libxfs-apply: ensure guilt import retains commit messages

libxfs-apply: ensure guilt import retains commit messages

The commit message attached to each patch is being filtered out
during the process of preparing it for import via guilt. This
results in commits without the corresponding commit message.
Massage the patch format to ensure that guilt imports the commit
message along with the code changes.

Also, older versions of filterdiff do not handle git diff metadata
correctly, so add a version check on filterdiff to make sure we use
a working version.

Signed-off-by: Dave Chinner <david@fromorbit.com>