Execute show repo command in repository directory

When cloning the repo for the first time, in "get" mode, the
darcs_patchcount() function is executed in the darcs-src/ directory
instead of the darcs-src/$REPO directory. We should ensure the darcs
command is called with --repodir set to the correct repository
directory.

Signed-off-by: Vikraman Choudhury <vikraman@gentoo.org>