viewdiff: stricter highlighting and linkification check
Sometimes senders draw ASCII tables and such which we
get fooled into attempting highlighting and diffstat
anchoring.
We now require 3 consecutive diff header lines:
/^--- /, /^\Q+++\E /, and /^@@ /
to enable diff highlighting (whether generated with git or not).
The presence of a line matching /^diff / is not sufficient or
even useful to us for highlighting diffs, since that could just
be part of a line-wrapped sentence.
However, we'll now check for the presence of a line matching
/^diff --git / before enabling diffstat anchors. Otherwise
cover letters for a patch series may fool us into creating
anchors for diffstats.