documentation-file-ref-check: Make git check work for multiple working directories

With multiple git working directories, '.git' may also be a text file
linking to the actual git tree instead of a directory.

Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Tested-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/20210603012806.331132-1-robh@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>