LinkageMonitor to support repository which uses non-SNAPSHOT in HEAD

[suztomo]

When applied to google-cloud-java (googleapis/google-cloud-java#5527), LinkageMonitor did not compare the result:

The content of the snapshot BOM and the original BOM are the same. Not running comparison.

The assumption that HEAD has "-SNAPSHOT" version is not always true. For example google-cloud-bigquery's HEAD has

   <version>0.97.0-alpha</version>

https://github.com/googleapis/google-cloud-java/blob/master/google-cloud-clients/google-cloud-bigquery/pom.xml

[suztomo]

It turned out this is unnecessary. The repository copy I picked up from google-cloud-java was in the middle of release and master temporarily had non-SNAPSHOT version.