merge: break out "both renamed a -> b" case
We can further limit the scope of the 2-way merge case by breaking out
the case where the file was not created from scratch on both sides but
rather renamed in the same way (and is therefore a 3-way merge). This
involves copying some code, but it makes it clearer which case the
"Note:" in the code refers to.
Testing that hghave does not crash when checking features
$ "$TESTDIR/hghave" --test-features 2>/dev/null