Never clean files in quilt status directory

Never clean files in quilt status directory

The top-level "distclean" and "mrproper" targets delete zero-size
files in the source, perhaps assumed to be stamp files.  This is fine
for actual source direcgtories, but disastrous for a quilt status
directory.

For each patch, quilt stores the previous versions of the modified
files in a subdirectory of .pc.  If the patch creates a file, this
file will be empty.  Removing it effectively removes the file creation
from the patch.

The .git directory is already excluded from the clean rules for
a similar reason, so do the same for .pc.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Gbp-Pq: Name never-clean-files-in-quilt-status-directory.patch