Re: 4-way diff (base,ours,theirs,merged) to review merge results

"Neal Kreitzinger" [off-list ref] writes:

....  What is the 
best way to display a 4-way diff of merge-base, "ours", "theirs", and 
"merged" after a merge completes so you can review the "merged" results for 
correctness?

Ahh, sorry.  While everything I wrote in my previous reply is correct with
respect to what happens _during_ a merge until you resolve it, I did not
realize that you were asking how to view a merge _after_ it is made.

For a two-parent merge $M, "git show --cc $M" runs a three-way diff
between $M (merge result), $M^1 (the first parent) and $M^2 (the other
parent) and the combined diff it shows is equivalent to:

  $ git diff --cc $M $M^1 $M^2

Notice the order of parameters. Unlike a normal "diff A B" to ask the
command to explain how the state B is reached from state A, you give the
result $M and ask the command to explain how it was reached from other
states.

So in a similar way, running

  $ git diff --cc $M $M^1 $M^2 $(git merge-base $M^1 $M^2)

should show a combined patch that explains the state at $M relative to the
states recorded in its parents and the merge base.

I've never tried it myself, though, as I never needed such an operation.

You can try a trivial example with 4d9e079, which merges 583c389 ec7ff5b
and has conflicts in cache.h

$ git show 4d9e079 -- cache.h
Output omitted; you can see it is the same as the next one for yourself.

$ git diff --cc 4d9e079 583c389 ec7ff5b -- cache.h
diff --cc cache.h
index 3a8e125,24732e6..422c5cf

--- a/cache.h
+++ b/cache.h

@@@ -1177,7 -1176,7 +1177,8 @@@ extern void setup_pager(void)
  extern const char *pager_program;
  extern int pager_in_use(void);
  extern int pager_use_color;
 +extern int term_columns(void);
+ extern int decimal_width(int);
  
  extern const char *editor_program;
  extern const char *askpass_program;

One side adds term_columns, the other side adds decimal_width.

$ git diff --cc 4d9e079 583c389 ec7ff5b \
    $(git merge-base 583c389 ec7ff5b) -- cache.h
diff --cc cache.h
index 3a8e125,24732e6,9bd8c2d..422c5cf

--- a/cache.h
+++ b/cache.h

@@@@ -1177,7 -1176,7 -1176,6 +1177,8 @@@@ extern void setup_pager(void)
   extern const char *pager_program;
   extern int pager_in_use(void);
   extern int pager_use_color;
 ++extern int term_columns(void);
+ +extern int decimal_width(int);
   
   extern const char *editor_program;
   extern const char *askpass_program;

The third column is a diff between $M and $(git merge-base $M^1 $M^2); the
resulting two new lines are indeed shown as additions against the merge
base.