completion: add '--copy' option to 'git branch'

In 52d59cc645 (branch: add a --copy (-c) option to go with --move (-m),
2017-06-18), `git branch` learned a `--copy` option.  Include it when
providing command completions.

Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1205,7 +1205,7 @@ _git_branch ()
 			--color --no-color --verbose --abbrev= --no-abbrev
 			--track --no-track --contains --no-contains --merged --no-merged
 			--set-upstream-to= --edit-description --list
-			--unset-upstream --delete --move --remotes
+			--unset-upstream --delete --move --copy --remotes
 			--column --no-column --sort= --points-at
 			"
 		;;