completion: expand "push --delete <remote> <ref>" for refs on that <remote>

Change the completion of "push --delete <remote> <ref>" to complete
refs on that <remote>, not all refs.

Before this cloning git.git and doing "git push --delete origin
p<TAB>" will complete nothing, since a fresh clone of git.git will
have no "pu" branch, whereas origin/p<TAB> will uselessly complete
origin/pu, but fully qualified references aren't accepted by
"--delete".

Now p<TAB> will complete as "pu". The completion of giving --delete
later, e.g. "git push origin --delete p<TAB>" remains unchanged, this
is a bug, but is a general existing limitation of the bash completion,
and not how git-push is documented, so I'm not fixing that case, but
adding a failing TODO test for it.

The testing code was supplied by SZEDER Gábor in
<20170421122832.24617-1-szeder.dev@gmail.com> with minor setup
modifications on my part.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: SZEDER Gábor <szeder.dev@gmail.com>
Test-code-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -709,6 +709,7 @@ __git_complete_remote_or_refspec ()
 		i="${words[c]}"
 		case "$i" in
 		--mirror) [ "$cmd" = "push" ] && no_complete_refspec=1 ;;
+		-d|--delete) [ "$cmd" = "push" ] && lhs=0 ;;
 		--all)
 			case "$cmd" in
 			push) no_complete_refspec=1 ;;

t/t9902-completion.sh

@@ -1457,4 +1457,38 @@ test_expect_failure 'complete with tilde expansion' '
 	test_completion "git add ~/tmp/" "~/tmp/file"
 '
 
+test_expect_success 'setup other remote for remote reference completion' '
+	git remote add other otherrepo &&
+	git fetch other
+'
+
+for flag in -d --delete
+do
+	test_expect_success "__git_complete_remote_or_refspec - push $flag other" '
+		sed -e "s/Z$//" >expected <<-EOF &&
+		master-in-other Z
+		EOF
+		(
+			words=(git push '$flag' other ma) &&
+			cword=${#words[@]} cur=${words[cword-1]} &&
+			__git_complete_remote_or_refspec &&
+			print_comp
+		) &&
+		test_cmp expected out
+	'
+
+	test_expect_failure "__git_complete_remote_or_refspec - push other $flag" '
+		sed -e "s/Z$//" >expected <<-EOF &&
+		master-in-other Z
+		EOF
+		(
+			words=(git push other '$flag' ma) &&
+			cword=${#words[@]} cur=${words[cword-1]} &&
+			__git_complete_remote_or_refspec &&
+			print_comp
+		) &&
+		test_cmp expected out
+	'
+done
+
 test_done