git: Import git-subtree at fc364c7

I need git-subtree from master because it's broken in git-1.8.5, the
latest release. In particular, this command is broken:
  git subtree add -P <prefix> <repository> <refspec>

Tags: public
Import-command: wget -O ~/bin/git-subtree\
https://git.kernel.org/cgit/git/git.git/\
plain/contrib/subtree/git-subtree.sh\
?id=d2446dfd7f3b3f8948142cfb07a0270e2497d93f