Merge branch 'jc/abort-ll-merge-with-a-signal'

When the external merge driver is killed by a signal, its output should not be trusted as a resolution with conflicts that is proposed by the driver, but the code did. * jc/abort-ll-merge-with-a-signal: t6406: skip "external merge driver getting killed by a signal" test on Windows ll-merge: killing the external merge driver aborts the merge