We will now populate the old_commit on demand, which will
allow us to keep going even if the given commit does not
exist locally.

Fixes #461

Adding support for git remote set-url/get-url API to Remote

Just went through all changes and adjusted them to the best of my
abilities. As there are no tests to claim otherwise, I believe
this is correct enough.

However, it becomes evident that it's no longer possible to just
make changes without backing them with a respective test.

`get-url` is a new API that is not widely available yet (introduced in git 2.7.0), and
provokes failure on travis.

Signed-off-by: Guyzmo <guyzmo+github@m0g.net>

Both commands enable handling of a little known feature
of git, which is to support multiple URL for one remote.
You can add multiple url using the `set_url` subcommand of
`git remote`. As listing them is also handy, there's a
nice method to do it, using `get_url`.

* adding set_url method that maps to the git remote set-url command¶
 * can be used to set an URL, or replace an URL with optional positional arg¶
 * can be used to add, delete URL with kwargs (matching set-url options)¶
* adding add_url, delete_url methods that wraps around set_url for conveniency¶
* adding urls property that yields an iterator over the setup urls for a remote¶
* adding a test suite that checks all use case scenarii of this added API.¶

Signed-off-by: Guyzmo <guyzmo+github@m0g.net>

This catches the case where the matched line contains "(" or ")"
characters.