patch library: don't use -p1 for git patches

patch library: don't use -p1 for git patches

For git patches, the patch library will automatically use
-p1 and strip the a/ b/ filename prefix, while for other
patches it uses -p0 and doesn't strip anything. We usually
use a/ b/ prefixes even when the patches aren't in the git
format and strip a path in gentree, so remove this from
the library.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>