pgm: Make --git-dir a string

pgm: Make --git-dir a string

DHT based repository types don't use a java.io.File to name the
repository.  Moving the type to a string starts to open up more types
of repository names, making the standard pgm package easier to reuse
on other storage systems.

Change-Id: I262ccc8c01cd6db88f832ef317b0e1e5db2d016a
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>