skip git clean if there is no .git directory
opennms-rrd-converter improvement.

As an optional step, you can now remove the old JRBs (single-metrics)
and the temporal files generated during the conversion.
opennms-rrd-converter improvement Part-II.

Re-implemented the method to find single-metric files. The candidates to
be removed are the files on which the file name is equal to the only one
single DS defined inside of it.