diff3: fix merge mode

Make the merge mode compatible with GNU diff3
Add tests for all the changes, those tests are extracted from the
etcupdate testsuite.

This version passes the etcupdate testsuit

diff3: fix merge mode

Make the merge mode compatible with GNU diff3
Add tests for all the changes, those tests are extracted from the
etcupdate testsuite.

This version passes the etcupdate testsuite and the diffutils diff3
test suite.

MFC After: 1 week

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