completion: add subcommand completion for rerere

Managing recorded resolutions requires command-line usage of git-rerere.
Added subcommand completion for rerere and path completion for its
subcommand forget.

Signed-off-by: Cornelius Weig <cornelius.weig@tngtech.com>
Reviewed-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -2401,6 +2401,17 @@ _git_replace ()
 	__gitcomp_nl "$(__git_refs)"
 }
 
+_git_rerere ()
+{
+	local subcommands="clear forget diff remaining status gc"
+	local subcommand="$(__git_find_on_cmdline "$subcommands")"
+	if test -z "$subcommand"
+	then
+		__gitcomp "$subcommands"
+		return
+	fi
+}
+
 _git_reset ()
 {
 	__git_has_doubledash && return