Re: merge-one-file: use common as base, instead of emptiness.

Petr Baudis [off-list ref] writes:

But obviously now the trouble is opposite, when the files are completely
unrelated, since now you likely get large conflicting areas interleaved
with some scarce common lines... And this might get to be a big PITA to
resolve as well.

Yeah, you can always count lines in $orig and $src2 after
two-file merge runs, and if it appears we do not have enough
common section just empty the $orig before running merge.