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

[bapt]

Summary

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

[emaste]

usr.bin/diff3/tests/Makefile

26–27

these files missing from review

[bapt]

add missing input files

[emaste]

Hmm, I already get matching output between diff3 in main and gdiff3 with -m and -E -m, e.g.

$ diff3 -m -L 1 -L 2 -L 3 out.txt fbsdid1.txt fbsdid2.txt
<<<<<<< 2
# $FreeBSD: head/local 12000 jhb $
=======
# $FreeBSD: head/local 12345 jhb $
>>>>>>> 3

this is a file

these are some local mods to the file

[emaste]

Oops, I actually had an old copy of GNU diff3 installed

[emaste]

It seems the functionality provided by -E isn't obvious from the (GNU diff3) man page, but this does provide parity with the implementation