gitk: Use the --submodule option for displaying diffs when available
gitk: Use the --submodule option for displaying diffs when available

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 <3a81d01482cca860e4077eb409d7db78ebdc6f24@web.de>
Signed-off-by: Paul Mackerras <19a0ba370c443ba08d20b5061586430ab449ee8c@samba.org>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git