Refactor repository construction to builder class

Refactor repository construction to builder class

The new FileRepositoryBuilder class helps applications to construct
a properly configured FileRepository, with properties assumed based
upon the standard Git rules for the local filesystem.

To better support simple command line applications, environment
variable handling and repository searching was moved into this
builder class.

The change gets rid of the ever-growing FileRepository constructor
variants, and the multitude of java.io.File typed parameters,
by using simple named setter methods.

Change-Id: I17e8e0392ad1dbf6a90a7eb49a6d809388d27e4c
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>