Allow pgm Main to be extended

Allow pgm Main to be extended

3rd party packages that use repository types other than FileRepository
may wish to extend our pgm package and implement their own resolution
scheme for repository "names" that are passed in by the --git-dir
command line option.  Make that possible by allowing the package to
extend the Main class and override the lookup.

This is primarily useful when developing new storage implementations
and trying to experiment with the results, without linking all of it
into the core JGit package.

Change-Id: Id30e168da16341e5da43365688a63aa30c7b7e2c
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>