Git.pm: Add support for subdirectories inside of working copies

Git.pm: Add support for subdirectories inside of working copies

This patch adds support for subdirectories inside of working copies;
you can specify them in the constructor either as the Directory
option (it will just get autodetected using rev-parse) or explicitly
using the WorkingSubdir option. This makes Git->repository() do the
exact same path setup and repository lookup as the Git porcelain
does.

This patch also introduces repo_path(), wc_path() and wc_subdir()
accessor methods and wc_chdir() mutator.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>