In general manpages are put into the eight numbered sections and for disambiguation may have additional subsection/extension specified, so eg. Perl modules have their manpages named /usr/share/man/man3/Foo::Bar.3pm.gz
Now, there are some things which are distribution specific.
Debian man has the -e option to select the subsection, so placing the git manpages in man1/foo.1git.gz would allow "man -e git foo" (and also "man 1git foo") to work, and of course "man foo" if this were the only foo to find.
Fedora man seems not to have anything like the -e option, but some manpages *are* placed in additional section directories there, so man1git/foo.1git.gz with "man 1git foo" would work there.
Of course none of these options looks as cool as man git foo.