git-completion.bash: complete branch.*.rebase as boolean

6fac1b8 (completion: add missing config variables, 2009-06-29) added
"rebase" to the list of completions for "branch.*.*", but forgot to
specify completions for the values that this configuration variable
can take (namely "false" and "true").  Fix this.

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

contrib/completion/git-completion.bash

@@ -1820,6 +1820,10 @@ _git_config ()
 		__gitcomp_nl "$(__git_refs)"
 		return
 		;;
+	branch.*.rebase)
+		__gitcomp "false true"
+		return
+		;;
 	remote.*.fetch)
 		local remote="${prev#remote.}"
 		remote="${remote%.fetch}"