diff3: fix diff3 -A

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.

Reviewed by: thj
Differential Revision: https://reviews.freebsd.org/D55276

(cherry picked from commit 8d442cf32e4fb29fbb85090ac29d32d1e30fd824)