git difftool does does not respect current working directory

Hello.

It is useful to compare the modified version of a file to HEAD's version, in
order to review changes before committing. gitk is fine for this but I often use
git difftool -t gvimdiff, so that I can rewrite code right into my diff editor.
Doing this, it is very likely that I open some more files (e.g. foo.h
corresponding to foo.c) in gvimdiff.
Unfortunately, 'git difftool' does not keep the current working directory while
launching gvimdiff.

=> Is it done on purpose ?
If not, it is probably a good idea to fix this.

I will be more than happy to contribute but I have some hard time to get
familiar with git source code.
Any help to locate the proper pieces of code would be realy appreciated.

--
Fred