worktree: new place for "git prune --worktrees"

Commit 23af91d (prune: strategies for linked checkouts - 2014-11-30)
adds "--worktrees" to "git prune" without realizing that "git prune" is
for object database only. This patch moves the same functionality to a
new command "git worktree".

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>

git.c

@@ -484,6 +484,7 @@ static struct cmd_struct commands[] = {
 	{ "verify-tag", cmd_verify_tag, RUN_SETUP },
 	{ "version", cmd_version },
 	{ "whatchanged", cmd_whatchanged, RUN_SETUP },
+	{ "worktree", cmd_worktree, RUN_SETUP },
 	{ "write-tree", cmd_write_tree, RUN_SETUP },
 };