commit: remove reverse search for copy source when not in parent (issue4476)
Previously, we had weird, nonsensical behavior when committing a file move
with a missing source. This removes that weird logic and tests that the bug
this strange behavior caused is fixed. Also adds a longish comment to prevent
some poor soul from accidentally re-implementing the bug in the future.
Testing that hghave does not crash when checking features
$ "$TESTDIR/hghave" --test-features 2>/dev/null