completion: replace core.abbrevguard to core.abbrev

The core.abbrevguard config variable had removed and
now core.abbrev has been used instead. Teach it.

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

contrib/completion/git-completion.bash

@@ -2132,7 +2132,7 @@ _git_config ()
 		color.ui
 		commit.status
 		commit.template
-		core.abbrevguard
+		core.abbrev
 		core.askpass
 		core.attributesfile
 		core.autocrlf