Store subtree metadata in .gittrees file

- Most of the changes are taken from helmo’s repository from following
URL:
https://github.com/helmo/git-subtree/blob/master/git-subtree.sh
- Add code to create a file named .gittrees which can store information
on the subtree that is getting merged to the super-repository. The
advantage is ‘subtree push’ and ‘subtree pull’ subcommands can be
called with only the —prefix option and no need to provide remote url
- Add tests for new feature additions. All existing and new tests pass
successfully
Files changed in this commit:
1. git/contrib/subtree/git-subtree.sh
2. git/contrib/subtree/t/t7900-subtree.sh