RFR: hg: allow tags without email

[Erik Helin]

Hi all,

please review this patch which allows `HgRepository.tag` to allow a `null` argument for `authorEmail`. Mercurial does
not require e-mails for authors, so neither should `HgRepository`.

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

Thanks,
Erik

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

Commit messages:
 - hg: allow tags without email

Changes: https://git.openjdk.java.net/skara/pull/729/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=729&range=00
  Stats: 18 lines in 2 files changed: 17 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/729.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/729/head:pull/729

PR: https://git.openjdk.java.net/skara/pull/729