index: Speed up loading a tree into the index

The index is empty; repeated tree entries cannot collide.

cc github/gitrpc#83
Added git_branch_name().

This is a convenience function to get the branch name of a given
ref. The returned branch name is compatible with the name that can
be supplied e.g. to git_branch_lookup(). That is, the prefixes
"refs/heads" or "refs/remotes" are omitted.

Also added a new test for testing the new function.