bash completion: complete CHERRY_PICK_HEAD

When e.g. in a failed cherry pick we did not recognize
CHERRY_PICK_HEAD as we do e.g. REBASE_HEAD in a failed rebase let's
rectify that.

When REBASE_HEAD was added in fbd7a23237 (rebase: introduce and use
pseudo-ref REBASE_HEAD, 2018-02-11) a completion was added for it, but
no corresponding completion existed for CHERRY_PICK_HEAD added in
d7e5c0cbfb (Introduce CHERRY_PICK_HEAD, 2011-02-19).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -744,7 +744,7 @@ __git_refs ()
 			track=""
 			;;
 		*)
-			for i in HEAD FETCH_HEAD ORIG_HEAD MERGE_HEAD REBASE_HEAD; do
+			for i in HEAD FETCH_HEAD ORIG_HEAD MERGE_HEAD REBASE_HEAD CHERRY_PICK_HEAD; do
 				case "$i" in
 				$match*)
 					if [ -e "$dir/$i" ]; then