RFR: 2084: Add a `/branch` command similar to `/tag` to create a new branch from a commit [v3]

[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

Erik Duveblad has updated the pull request incrementally with one additional commit since the last revision:

  Use null free Optional pattern
  
  Co-authored-by: Erik Joelsson <37597443+erikj79 at users.noreply.github.com>

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Changes:
  - all: https://git.openjdk.org/skara/pull/1583/files
  - new: https://git.openjdk.org/skara/pull/1583/files/d32d69ba..44d9b3cb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1583&range=02
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1583&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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