Moves diff_chunk_type into the public diff API.

This brings in a build fix for OpenBSD/sparc64.

This removes the gcc ternary if extension (includes follow-up diff.git fix for missing '\' in wrapped macro 7a38b3a33f8b124c9c01da154dfdf711d328b01d)

ok stsp@

This new diff implementation was started by Neels Hofmeyr during the u2k20 hackathon and now replaces diffreg.c code lifted from the OpenBSD base system. The integration of this code into Got was done by me. Got now uses the patience diff algorithm by default. The diff.git repository will remain the primary repository for the diff code, which already compiles and runs on other operating systems such as Linux. Any fixes and improvements for files inherited from the diff.git repository should be written against that repository and synced to got.git afterwards.