Re: On recording renames

Junio C Hamano [off-list ref] writes:

       a1--a2--a3--a4--a5--A
      /                     \
  ---O---b1--b2--b3--b4--B---* 

    O has hello.c
    a1 renames file hello.c to world.c and a2-a5-A modifies world.c
    b1-b4-B modifies hello.c 
    we are about to merge A and B

    comparing O and A may not notice O's hello.c and A's world.c
    are similar!

But you are allowed to write a new merge strategy that is more
careful about renames.  There is no reason you can only look at
three trees.  Such a merge strategy, when given commit A and B,
would walk the history back, running "diff-tree -M" for each
commit along the way, and difference between O's hello.c and
a1's world.c would be hopefully *much* smaller than O's hello.c
and A's world.c -- even the current similarity estimator may
recognize it is a rename.

A bit more on merges and renames.  The thread that started on
Dec 16 2005 by Don Zickus is about a case that anybody
interested in renaming merge should think about.  Unfortunately
gmane web interface says it is "down for maintenance" so I
cannot give an URL, but the message IDs of key messages are:

        [ref]
        [ref]

The whole thread is worth reading, but the punch line is:

        The transition happened over time with multiple commits.
        You cannot record "this is the rename" by attributing that
        information to one particular commit.