Add RepoClient as wrapper that auto-detects and supports both git & hg

[eemeli]

This builds on top of #5, so at least currently these are the only proper changes here.

A new RepoClient class is added, with a minimal set of methods as required by the rest of the library for repository operations. A Mercurial repo is detected from the presence of a .hg directory, and for Git we look for .git and check that git rev-parse --is-inside-work-tree returns true.

The hglib stuff should work just as before; for git we don't need any wrapper library as the command line is plenty. For the blame I did need to write a minimal parser, but that was only about 10 lines of code.

The Blame().handleFile() function signature changed a bit through all this, but AFAIK that has no external users.

[flodolo]

I wonder if we could have the "apply black formatting" as a completely separate PRs? It makes reviewing unnecessarily hard, even with separate commits.

I'd be OK with you landing that change directly without review, since that's the standard we use all over the place.

[eemeli]

I'd be OK with you landing that change directly without review, since that's the standard we use all over the place.

Done, and rebased also this PR.

[eemeli]

Rebased as #5 was merged. Also exposed git() as a utility function separately from RepoClient + added head() and log() methods to support usage from test_fluent_migrations in m-c.

[bcolsson]

Reviewed this and nothing stands out to me so approving. Next time there's a migration ready I'll attempt a test with this.

[flodolo]

Have you considered (and discarded) the idea of using a library similar to hglib for git?

[eemeli]

Yes. This way we avoid a layer of indirection in needing to refer to an external definition for what git() does, and have instead this minimal six-line function that's sufficient for our purposes.