pq: rewrite patch export functionality

Use our own function for constructing the patch files instead of using
the format-patch command of git. This way, we get the desired output
format directly, without the need for the error-prone "format-patch,
parse patch files, mangle and re-write patch files" cycle.

Also, fix patch naming in patch generation when '--no-patch-numbers' is
used.  Previously, multiple commits with the same subject resulted in
multiple patches having the same filename. This lead into broken series
with missing patches as patch files were overwritten by the topmost
commit.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>