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: NShawn O. Pearce <spearce@spearce.org>