git-gui: Track our own embedded values and rebuild when they change
Like core-Git we now track the values that we embed into our shell script wrapper, and we "recompile" that wrapper if they are changed. This concept was lifted from git.git's Makefile, where a similar thing was done by Eygene Ryabinkin. Too bad it wasn't just done here in git-gui from the beginning, as the git.git Makefile support for GIT-GUI-VARS was really just because git-gui doesn't do it on its own.
Signed-off-by: Shawn O. Pearce <firstname.lastname@example.org>