Edit maven.md

See merge request !123

In #83 it was highlighted that commitlint did not behave gracefully for
MRs which do not branch from the current tip of the default branch. This
was because, ultimately, `git merge-base --fork-point` exited with a
non-zero exit code which caused the job to fail.

Gracefully handle this case by catching the exit status and falling back
to checking all of the commit history.

Additionally, while `git merge-base` does appear to match the algorithm
[used by GitLab itself][1] the use of `--fork-point` fails if the branch
was not branched from the tip of default branch. Removing `--fork-point`
better matches the algorithm used by GitLab itself.

[1]: !64 (comment 523905)

Closes #83

fix(commitlint): behave gracefully if no common ancestor commit is found

Closes #83

See merge request !124