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>
diff --git a/Makefile b/Makefile
index 827006b..bfdc908 100644
--- a/Makefile
+++ b/Makefile
@@ -886,6 +886,7 @@
 BUILTIN_OBJS += builtin/verify-commit.o
 BUILTIN_OBJS += builtin/verify-pack.o
 BUILTIN_OBJS += builtin/verify-tag.o
+BUILTIN_OBJS += builtin/worktree.o
 BUILTIN_OBJS += builtin/write-tree.o
 
 GITLIBS = $(LIB_FILE) $(XDIFF_LIB)