The previous implementation assumed a .git repository be present.  This
is obviously not true for release tarballs.  In that case, gitversion.sh
emits git's fatal warnings that a repository was not found, the revision
is set to the empty string, and the smoke test crashes in version.cpp.

Add a test in gitversion.sh to gracefully handle check for a git
repository being present, and modify version.cpp to handle empty
revision strings.

Reported-by: Peter van Hoof