diff3: make the diff3 -E -m and diff3 -m behaviour match gnu diff3

In gnu diff3 3 way merging files where the new file and the target are
already the same will die and show what has failed to be merged except
if -E is passed in argument, in this case it will finish the merge.

This difference in behaviour was breaking one of the etcupdate testcase
with bsd diff3

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D41730