diff3: fix diff3 -A

[bapt]

Summary

for cases where file2 differs but file1 and file3 agrees, the code
stored the file2 and file3 line numbers in the de[] editing script
entries but used them as if they were file1 line numbers.

[thj]

This looks ok, but I will have to admit that diff3 was always a bit of a mystery box. Please update the comment, but assuming tests don't start failing I'm willing to bet this is an improvement.

usr.bin/diff3/diff3.c

884

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