RFR: vcs: add method ReadOnlyRepository.follow

[Erik Helin]

Hi all,

please review this patch that adds the method `ReadOnlyRepository.follow` for getting all commits that changed a
particular file.

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

Thanks,
Erik

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

Commit messages:
 - vcs: add method ReadOnlyRepository.follow

Changes: https://git.openjdk.java.net/skara/pull/740/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=740&range=00
  Stats: 131 lines in 4 files changed: 103 ins; 26 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/740.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/740/head:pull/740

PR: https://git.openjdk.java.net/skara/pull/740