Base class to support constructing a org.eclipse.jgit.lib.Repository.

Applications must set one of setGitDir(File) or setWorkTree(File), or use readEnvironment() or findGitDir() in order to configure the minimum property set necessary to open a repository.

Single repository applications trying to be compatible with other Git implementations are encouraged to use a model such as:

new RepositoryBuilder() //
		.setGitDir(gitDirArgument) // --git-dir if supplied, no-op if null
		.readEnviroment() // scan environment GIT_* variables
		.findGitDir() // scan up the file system tree
		.build()