completion: add --option completion for most builtin commands

Many builtin commands use parseopt which can expose the option list
via --git-completion-helper but do not have explicit support in
git-completion.bash. This patch detects those commands and uses
__gitcomp_builtin for option completion.

This does not pollute the command name completion though. "git <tab>"
will show you the same set as before. This only kicks in when you type
the correct command name.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -3035,12 +3035,40 @@ _git_worktree ()
 	fi
 }
 
+__git_complete_common () {
+	local command="$1"
+
+	case "$cur" in
+	--*)
+		__gitcomp_builtin "$command"
+		;;
+	esac
+}
+
+__git_cmds_with_parseopt_helper=
+__git_support_parseopt_helper () {
+	test -n "$__git_cmds_with_parseopt_helper" ||
+		__git_cmds_with_parseopt_helper="$(__git --list-parseopt-builtins)"
+
+	case " $__git_cmds_with_parseopt_helper " in
+	*" $1 "*)
+		return 0
+		;;
+	*)
+		return 1
+		;;
+	esac
+}
+
 __git_complete_command () {
 	local command="$1"
 	local completion_func="_git_${command//-/_}"
 	if declare -f $completion_func >/dev/null 2>/dev/null; then
 		$completion_func
 		return 0
+	elif __git_support_parseopt_helper "$command"; then
+		__git_complete_common "$command"
+		return 0
 	else
 		return 1
 	fi

t/t9902-completion.sh

@@ -1454,6 +1454,12 @@ test_expect_success 'completion used <cmd> completion for alias: !f() { : git <c
 	EOF
 '
 
+test_expect_success 'completion without explicit _git_xxx function' '
+	test_completion "git version --" <<-\EOF
+	--build-options Z
+	EOF
+'
+
 test_expect_failure 'complete with tilde expansion' '
 	git init tmp && cd tmp &&
 	test_when_finished "cd .. && rm -rf tmp" &&