Fixed corner case in trends metric.
Merge branch 'master-patch'
[scm] Add Revisions metric

This metric is a subset of the Commits metric. At some point those should
be integrated into the same class. This class counts unique revisions across
all of the repositories.
Merge branch 'master' into scm_query_builder
Fix conflicts in example

Conflicts:
	vizgrimoire/metrics/scm_metrics.py