re-running merge on a single file

[Chris Packham <judge.packham@gmail.com>]

Hi List,

I'm in the middle of updating our copy of another projects repository.
So far its been OK, we haven't diverged too much so it was a fairly
simple operation to add the projects repository as a 'vendor' remote
and get git merge to do most of the heavy lifting then use git merge
tool to fix up the things that git merge couldn't resolve. So right
now I have a history something like this.

-o-o-o-o-o           master
 \           \
  \           m        update
   \         /
    \-o-o-o            vendor

Now I'm finding that when I first used git mergetool for some things I
messed up and need to fix the code. In some cases this is because some
naming schemes changed in the vendor branch so I need to make
equivalent changes to the code we've added, no problem thats just more
commits on the update branch. In other cases I'm actually thinking
that it'd be easier to ask git to try the merge again on that one
file. I can ask git difftool to show me the pre-merge differences with
'git difftool master..vendor <file>' but what I'd really like to do is
ask git mergetool to do something similar so I can re-do my manual
merging.

Alternatively I could manually drive and external merge tool like
kdiff3 if I had a recipe for getting the common ancestor revision.
Something like

git checkout <magic pathspec> -- file
mv file file.base

git checkout master -- file
mv file file.ours

git checkout vendor -- file
mv file file.theirs

kdiff3 file.base file.ours file.theirs -o file

---
Thanks,
Chris