add support for custom merge messages without 'TargetBranch' capture group

[Dobriy33]

Hello. There is small issue, that doesn't allow to use GV in bitbucket cloud repositories. Bitbucket provides following merge message format for pull requests: "Merged in feature/one (pull request #N)". This message doesn't match the requirements for custom branches because doesn't contain TargetBranch

The regular expression should contain the following capture groups:
SourceBranch - Identifies the source branch of the merge
TargetBranch - Identifies the target of the merge
PullRequestNumber - Captures the pull-request number

I passed current branch name to mergeMessage processor to get it if message regex doesn't contain Target branch.
I tested it on Bitbucket project and it works good.

[asbjornu]

Can't you use the merge-message-formats introduced in #1488?

[Dobriy33]

Oh.. I checked code again and I see, that TargetBranch isn't used anywhere besides tests.. But configuration.md says that this capture group is required in regex..
I think we can close this PR.