re-running merge on a single file

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