Merge pull request #84 from mniemela/fixes

Imported CommandError to extensions/FileRevs.py
[Metrics] Ignore actions that do no alter metric values

These are 'delete' (D) and 'move' (actions).
This commit also fixes #85.
Metrics-Extension: Fixed AttributeError: 'NoneType' object has no
attribute 'strip'

This is similar to commit 34525e927ffb31795b45145dcddf29fa20675d96. In
new versions path can be None, so I added an if block to check if that
is the case, and return if so.