Hide mvn output for checkAvailable

The output of the mvn run used to check if an artifact is available
in the m2 repos is not useful most of the time, and the possible failure
output can be misleading for people who don't know the internals of
the script.
The output is know put in target/log-extra.txt, and not display to the
user except in DEBUG mode.
Hardens git support

Look for FOLDER/.git instead of only FOLDER to see if a local clone
of a project exists. To handle the case when the .git folder has been
delete by mistake.
Merge pull request #26 from skyluc/pr-scala/cleanup-and-git

Tweaks on git support and checkAvailability