completion: teach rebase to use __gitcomp_builtin

Currently, _git_rebase() uses a hardcoded list of options for its
completion. However, we can use __gitcomp_builtin() to get a dynamically
generated list of completions instead.

Teach _git_rebase() to use __gitcomp_builtin() so that newly implemented
options in rebase will be automatically completed without any mucking
around in git-completion.bash.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -2004,15 +2004,18 @@ _git_range_diff ()
 	__git_complete_revlist
 }
 
+__git_rebase_inprogress_options="--continue --skip --abort --quit --show-current-patch"
+__git_rebase_interactive_inprogress_options="$__git_rebase_inprogress_options --edit-todo"
+
 _git_rebase ()
 {
 	__git_find_repo_path
 	if [ -f "$__git_repo_path"/rebase-merge/interactive ]; then
-		__gitcomp "--continue --skip --abort --quit --edit-todo --show-current-patch"
+		__gitcomp "$__git_rebase_interactive_inprogress_options"
 		return
 	elif [ -d "$__git_repo_path"/rebase-apply ] || \
 	     [ -d "$__git_repo_path"/rebase-merge ]; then
-		__gitcomp "--continue --skip --abort --quit --show-current-patch"
+		__gitcomp "$__git_rebase_inprogress_options"
 		return
 	fi
 	__git_complete_strategy && return
@@ -2022,19 +2025,8 @@ _git_rebase ()
 		return
 		;;
 	--*)
-		__gitcomp "
-			--onto --merge --strategy --interactive
-			--rebase-merges --preserve-merges --stat --no-stat
-			--committer-date-is-author-date --ignore-date
-			--ignore-whitespace --whitespace=
-			--autosquash --no-autosquash
-			--fork-point --no-fork-point
-			--autostash --no-autostash
-			--verify --no-verify
-			--keep-empty --root --force-rebase --no-ff
-			--rerere-autoupdate
-			--exec
-			"
+		__gitcomp_builtin rebase "" \
+			"$__git_rebase_interactive_inprogress_options"
 
 		return
 	esac