oeqa/selftest/devtool: Correct git clone of local repository

oeqa/selftest/devtool: Correct git clone of local repository

If the build environment is setup using `repo`, then poky/.git/object
is a symbolic link rather than a directory. To clone such repositories,
the source path must be prefixed with "file://". This avoids the
following error:

  fatal: failed to start iterator over '.../poky/.git/objects': Not a directory

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>