How best to set grep command based on current directory?

[Michael Welsh Duggan]

I would like to change the program invocation in "M-x grep" (or maybe
"M-x grep-find") to default to "git grep" when in directories that are
in git trees.  I could hack away at the grep.el sources to do this, or I
could write some advice around the grep command to do this, but I want
to ask here, first, if anyone believes that there is a more principled
way to make this change.

-- 
Michael Welsh Duggan
(md5i@md5i.com)