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 <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: artagnon

contrib/completion/git-completion.bash

@@ -1859,6 +1859,10 @@ _git_config ()
 			"
 		return
 		;;
+	diff.submodule)
+		__gitcomp "log short"
+		return
+		;;
 	help.format)
 		__gitcomp "man info web html"
 		return
@@ -2061,6 +2065,7 @@ _git_config ()
 		diff.renameLimit
 		diff.renames
 		diff.statGraphWidth
+		diff.submodule
 		diff.suppressBlankEmpty
 		diff.tool
 		diff.wordRegex