RFR: vcs: improve git->hg converter

[Erik Helin]

Hi all,

please review this patch that significantly improves the `GitToHgConverter`. The new converter handles all OpenJDK Git
repositories and also handles additional Git tags. I also improved the testing by quite a bit, the verification is much
more strict and I added a bunch of more tests. I also added some `@Disabled` tests (since they require an internet
connection), but they are still useful for running locally when working on the converter.

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

Thanks,
Erik

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

Commit messages:
 - Remove trailing whitespace
 - vcs: improve GitToHgConverter

Changes: https://git.openjdk.java.net/skara/pull/736/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=736&range=00
  Stats: 379 lines in 3 files changed: 232 ins; 61 del; 86 mod
  Patch: https://git.openjdk.java.net/skara/pull/736.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/736/head:pull/736

PR: https://git.openjdk.java.net/skara/pull/736