completion: use __gitcomp_builtin in _git_am

The new completable options are:

--directory
--exclude
--gpg-sign
--include
--keep-cr
--keep-non-patch
--message-id
--no-keep-cr
--patch-format
--quiet
--reject
--resolvemsg=

In-progress options like --continue will be part of --git-completion-helper
then filtered out by _git_am() unless the operation is in progress. This
helps keep marking of these operations in just one place.

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

contrib/completion/git-completion.bash

@@ -1105,12 +1105,13 @@ __git_count_arguments ()
 }
 
 __git_whitespacelist="nowarn warn error error-all fix"
+__git_am_inprogress_options="--skip --continue --resolved --abort"
 
 _git_am ()
 {
 	__git_find_repo_path
 	if [ -d "$__git_repo_path"/rebase-apply ]; then
-		__gitcomp "--skip --continue --resolved --abort"
+		__gitcomp "$__git_am_inprogress_options"
 		return
 	fi
 	case "$cur" in
@@ -1119,12 +1120,8 @@ _git_am ()
 		return
 		;;
 	--*)
-		__gitcomp "
-			--3way --committer-date-is-author-date --ignore-date
-			--ignore-whitespace --ignore-space-change
-			--interactive --keep --no-utf8 --signoff --utf8
-			--whitespace= --scissors
-			"
+		__gitcomp_builtin am "--no-utf8" \
+			"$__git_am_inprogress_options"
 		return
 	esac
 }