git-completion.bash: add diff.submodule to config list
git-completion.bash: add diff.submodule to config list

c47ef57 (diff: introduce diff.submodule configuration variable,
2012-11-13) introduced the diff.submodule configuration variable, but
forgot to teach git-completion.bash about it.  Fix this.

Signed-off-by: Ramkumar Ramachandra <5c355d1d38066ed0c5de420d67c1b5ed0687c4bd@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

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