Re: [RFC PATCH] mergetools/vimdiff: add vimdiff4 merge tool variant

Thanks David and Juno for your feedback.

I completely agree that adding another vimdiffX variant is ... not elegant.

So I've been thinking a bit more about how this whole "vim layout" mechanism can
be made more generic and this is what I came up with:

  1. Let's add a new configuration variable to the "vimdiff" merge tool called
     "layout":

         [mergetool "vimdiff"]
         layout = ...

  2. If this new variable is *not* present, vim will behave in the same way it
     does today (ie. a top row with the local, base and remote buffers, and a
     bottom row with the merged buffer).

  3. In all other cases, the contents of the "layout" variable will be
     intepreted following these rules:

       - ";" is used to separate "tab descriptors"
       - "," is used to separate "row descriptors"
       - "|" is used to separate "column descriptors"
       - "+" is used to load buffers that won't be displayed by default

     This will be better understood with some examples that emulate the behavior
     of the current "vimdiff", "vimdiff1", "vimdiff2" and "vimdiff3" variants as
     well as the proposed "vimdiff4" one:


       vimdiff  --> layout = "LOCAL | BASE | REMOTE, MERGED"

           ------------------------------------------
           |             |           |              |
           |   LOCAL     |   BASE    |   REMOTE     |
           |             |           |              |
           ------------------------------------------
           |                                        |
           |                MERGED                  |
           |                                        |
           ------------------------------------------


       vimdiff1 --> layout = "LOCAL* | REMOTE"

           ------------------------------------------
           |                   |                    |
           |                   |                    |
           |                   |                    |
           |     LOCAL         |    REMOTE          |
           |                   |                    |
           |                   |                    |
           |                   |                    |
           ------------------------------------------

           NOTE: In this case (where there is no "MERGED"
           buffer specified in the "layout" string), a "*"
           is needed to indicate which file will be the one
           containing the final version of the file after
           resolving conflicts.


       vimdiff2 --> layout = "LOCAL | MERGED | REMOTE"

           ------------------------------------------
           |             |           |              |
           |             |           |              |
           |             |           |              |
           |   LOCAL     |   BASE    |   REMOTE     |
           |             |           |              |
           |             |           |              |
           |             |           |              |
           |             |           |              |
           ------------------------------------------


       vimdiff3 --> layout = "LOCAL + REMOTE + BASE + MERGED"

           ------------------------------------------
           |                                        |
           |                                        |
           |                                        |
           |               MERGED                   |
           |                                        |
           |                                        |
           |                                        |
           ------------------------------------------

           NOTE: LOCAL, REMOTE and BASE are loaded as hidden
           buffers and you need to recall them explicitely.


       vimdiff4 --> layout = "BASE | LOCAL | REMOTE, MERGED; BASE | LOCAL; BASE | REMOTE"
 
           ------------------------------------------
           | <TAB #1> |  TAB #2  |  TAB #3  |       |
           ------------------------------------------
           |             |           |              |
           |   LOCAL     |   BASE    |   REMOTE     |
           |             |           |              |
           ------------------------------------------
           |                                        |
           |                MERGED                  |
           |                                        |
           ------------------------------------------

           ------------------------------------------
           |  TAB #1  | <TAB #2> |  TAB #3  |       |
           ------------------------------------------
           |                   |                    |
           |                   |                    |
           |                   |                    |
           |     BASE          |    LOCAL           |
           |                   |                    |
           |                   |                    |
           |                   |                    |
           ------------------------------------------

           ------------------------------------------
           |  TAB #1  |  TAB #2  | <TAB #3> |       |
           ------------------------------------------
           |                   |                    |
           |                   |                    |
           |                   |                    |
           |     BASE          |    REMOTE          |
           |                   |                    |
           |                   |                    |
           |                   |                    |
           ------------------------------------------


The nice thing about this approach is that, as we have seen, it is generic
enough to rule all current variants obsolete.

So, please let me know what you think about this:

  * Do you like this approach? Or am I trying to crack a nut with a sledgehammer
    by making the whole thing too complex?

  * In case you like it, should we keep the old "vimdiff1", "vimdiff2" and
    "vimdiff3" variants for backwards compatibility?
    If the answer is "yes", I'll just alias them to the new "layout" mechanism
    so that the amount of extra code needed for supporting them is minimal.

If you tell me you like this proposal, I'll go ahead and implement a patch for
all of this, taking into consideration David's suggestions for avoiding problems
with file with spaces in their names and also adding new documentation for all
of this.

    NOTE: The only non-trivial thing about implementing this is how to parse the
    "layout" variable syntax *in bash* to convert it into a sequence of vim
    commands that achieves the expected outcome... but seems like a funny
    weekend project :)

If you think it is not worth the effort, let me know if it is OK to just add
"vimdiff4" + documentation instead for now (or something else).

Thanks.