completion: use __gitcomp_nl() for completing refs

We do that almost everywhere, because it's faster for large number of
refs, see a31e62629 (completion: optimize refs completion, 2011-10-15).
These were the last two places where we still used __gitcomp() for
completing refs.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -980,7 +980,7 @@ _git_branch ()
 
 	case "$cur" in
 	--set-upstream-to=*)
-		__gitcomp "$(__git_refs)" "" "${cur##--set-upstream-to=}"
+		__gitcomp_nl "$(__git_refs)" "" "${cur##--set-upstream-to=}"
 		;;
 	--*)
 		__gitcomp "
@@ -1048,7 +1048,7 @@ _git_checkout ()
 
 _git_cherry ()
 {
-	__gitcomp "$(__git_refs)"
+	__gitcomp_nl "$(__git_refs)"
 }
 
 _git_cherry_pick ()