Time to change pg_regress diffs to unified by default?

Hi,

I personally, and I know of a bunch of other regular contributors, find
context diffs very hard to read. Besides general dislike, for things
like regression test output context diffs are just not well suited.
E.g. in
https://buildfarm.postgresql.org/cgi-bin/show_stage_log.pl?nm=prairiedog&dt=2017-04-06%2021%3A10%3A56&stg=check
the salient point (ERROR: 50 is outside the valid range for parameter "effective_io_concurrency" (0 .. 0))
is 130 lines into the diff, whereas it's right at the start in a unified diff.
Issues with one error that causes a lot of followup output changes are
really common in our regression suite.

I personally have PG_REGRESS_DIFF_OPTS set to -dU10, but that doesn't
help much analyzing buildfarm output.

Therefore I propose changing the defaults in pg_regress.c.

Greetings,

Andres Freund