Support "git worktree"

Author: Yasuo Ohgaki

vcsclean

@@ -1,6 +1,6 @@
 #! /bin/sh
 
-if test -d '.git'; then
+if test -d '.git' -o -f '.git'; then
     ${MAKE:-make} -f build/build.mk gitclean-work
 else
     echo "Can't figure out your VCS, not cleaning."