completion: fix branch.autosetup(merge|rebase)

When attempting to complete

  $ git config branch.auto<TAB>

'autosetupmerge' and 'autosetuprebase' don't come up. This is because
"$cur" is matched with "branch.*" and a list of branches are
completed. Add 'autosetupmerge', 'autosetuprebase' as candidates for
completion too, using __gitcomp_nl_append ().

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

contrib/completion/git-completion.bash

@@ -1841,6 +1841,7 @@ _git_config ()
 	branch.*)
 		local pfx="${cur%.*}." cur_="${cur#*.}"
 		__gitcomp_nl "$(__git_heads)" "$pfx" "$cur_" "."
+		__gitcomp_nl_append $'autosetupmerge\nautosetuprebase\n' "$pfx" "$cur_"
 		return
 		;;
 	guitool.*.*)