When displaying diffs in a submodule, this makes gitk display the
headlines of the commits being diffed, instead of just showing
not-quite-helpful SHA-1 pairs, if the underlying git installation
supports this. That makes it much easier to evaluate the changes, as
it eliminates the need to start a gitk inside the submodule and use
the superprojects hashes there to find out what the commits are about.
Since the --submodule option of git diff is new in git version 1.6.6,
this only uses the --submodule option when a git version of 1.6.6 or
higher is detected.
Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de> Signed-off-by: Paul Mackerras <paulus@samba.org>