Use libgit instead of sh commands

[timabell]

Probably not important, but currently just calling git command and capturing output in order to grab urls from config.

https://github.com/rust-lang/git2-rs - got a compile error including this:

tim@max:~/repo/gitopolis(url*)
$ cb
    Updating crates.io index
error: failed to select a version for the requirement `libgit2-sys = "^0.13.4"`
candidate versions found which didn't match: 0.13.2+1.4.2, 0.13.1+1.4.2, 0.13.0+1.4.1, ...
location searched: crates.io index
required by package `git2 v0.14.4`
    ... which satisfies dependency `git2 = "^0.14.4"` of package `gitopolis v0.1.0 (/home/tim/repo/gitopolis)`