MFC r351836, r351866, r354328: patch(1) /dev/null testing+improvement

MFC r351836, r351866, r354328: patch(1) /dev/null testing+improvement

r351836: patch(1): add some basic tests

Summary:
- basic: test application of patches created by diff -u at the
  beginning/middle/end of file, which have differing amounts of context
  before and after chunks being added
- limited_ctx: stems from PR 74127 in which a rogue line was getting added
  when the patch should have been rejected. Similar behavior was
  reproducible with larger contexts near the beginning/end of a file. See
  r326084 for details
- file_creation: patch sourced from /dev/null should create the file
- file_nodupe: said patch sourced from /dev/null shouldn't dupe the contents
  when re-applied (personal vendetta, WIP, see comment)
- file_removal: this follows from nodupe; the reverse of a patch sourced
  from /dev/null is most naturally deleting the file, as is expected based
  on GNU patch behavior (WIP)

r351866: patch(1): fix the file removal test, strengthen it a bit

To remain compatible with GNU patch, we should ensure that once we're
removing empty files after a reversed /dev/null patch we don't remove files
that have been modified. GNU patch leaves these intact and just reverses the
hunk that created the file, effectively implying --remove-empty-files for
reversed /dev/null patches.

r354328: patch(1): give /dev/null patches special treatment

We have a bad habit of duplicating contents of files that are sourced from
/dev/null and applied more than once... take the more sane (in most ways)
GNU route and complain if the file exists and offer reversal options.

This still falls short a little bit as selecting "don't reverse, apply
anyway" will still give you duplicated file contents. There's probably other
issues as well, but awareness is the first step to happiness.