RFR: bots: add checkout bot

[Erik Helin]

Hi all,

please review this patch that adds a new bot called "checkout". The bot can checkout a remote Git repository to a local
Hg repository using the recently improved `GitToHgConverter`. I also added two new unit tests for verifying the
functionality.

Testing:
- [x] `make test` passes on Linux x64
- [x] Added two new unit tests

Thanks,
Erik

-------------

Commit messages:
 - bots: add checkout bot

Changes: https://git.openjdk.java.net/skara/pull/743/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=743&range=00
  Stats: 429 lines in 8 files changed: 423 ins; 1 del; 5 mod
  Patch: https://git.openjdk.java.net/skara/pull/743.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/743/head:pull/743

PR: https://git.openjdk.java.net/skara/pull/743