List subcmd and modify push/pull to use .gittrees

- Add subtree list sub-command
- git subtree list - Lists the subtrees in current project
- Changes taken from helmo’s repository from following URL:
https://github.com/helmo/git-subtree/blob/master/git-subtree.sh
- Add tests for subtree list and subtree push/pull using .gittrees
- Files changed in this commit:
1. git/contrib/subtree/git-subtree.sh
2. git/contrib/subtree/t/t7900-subtree.sh