RFR: 2084: Add a `/branch` command similar to `/tag` to create a new branch from a commit

[Erik Duveblad]

Hi all,

please review this patch that adds a `/branch` commit command. The `/branch` commit command allows an integrator to create a _new_ branch in a repository by adding the comment `/branch <name>` on a commit's web page. The command will check that no branch with the given name exists (i.e. it won't update a branch with that name to that commit).

The implementation is very similar to the `/tag` command, but I don't think it makes sense to try to share code between them. They are both only around 60 lines of code, so trying to share code won't give us much.

I also added a couple of new unit tests.

Thanks,
Erik

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Commit messages:
 - skara-2084

Changes: https://git.openjdk.org/skara/pull/1583/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1583&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2084
  Stats: 445 lines in 4 files changed: 445 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/skara/pull/1583.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1583/head:pull/1583

PR: https://git.openjdk.org/skara/pull/1583