suggested by naddy

ok naddy

This moves the pthread_cond_destroy(need_commits) from stop_log_thread(), which can be called twice, to close_log_view(), which is called once. It also destroys the commit_loaded condition variable, which is created in open_log_view() but was never destroyed. ok stsp

ok naddy@

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.