Store subtree information in ini style format

Entries for push and pull are created to allow the case
where push must be to a different repo and/or branch than
the pull.

e.g.:

[subtree "<prefix>"]
	repository-pull = <repository>
	branch-pull = <branch>
	repository-push = <repository>
	branch-push = <branch>

Signed-off-by: Paul Campbell <pcampbell@kemitix.net>