libdiff: Detect and recover from file truncation.

If a memory-mapped file is truncated before we get to the end, the
atomizer may catch SIGBUS. Detect that, reduce the input length to
what we were actually able to read, and set a flag so the caller can
take further action (e.g. warn the user and / or start over).

Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D45217

Add 'contrib/libdiff/' from commit '9eb461aa4b61ab47855b2cee9e5b626a76888b5e'

git-subtree-dir: contrib/libdiff
git-subtree-mainline: f6d489f402c320f1a6eaa473491a0b8c3878113e
git-subtree-split: 9eb461aa4b61ab47855b2cee9e5b626a76888b5e

Reviewed by: imp
Sponsored by: Klara, Inc.