check-ignore: allow incremental streaming of queries via --stdin

Some callers, such as the git-annex web assistant, find it useful to
invoke git check-ignore as a persistent background process, which can
then have queries fed to its STDIN at any point, and the corresponding
response consumed from its STDOUT.  For this we need to invoke
check_ignore() once per line of standard input, and flush standard
output after each result.

The above use case suggests that empty STDIN is actually a reasonable
scenario (e.g. when the caller doesn't know in advance whether any
queries need to be fed to the background process until after it's
already started), so we make the minor behavioural change that "no
pathspec given." is no longer emitted in when STDIN is empty.

Even though check_ignore() could now be changed to operate on a single
pathspec, we keep it operating on an array of pathspecs since that is
a more convenient way of consuming the existing pathspec API.

Signed-off-by: Adam Spiers <git@adamspiers.org>
Documentation: add caveats about I/O buffering for check-{attr,ignore}

check-attr and check-ignore have the potential to deadlock callers
which do not read back the output in real-time.  For example, if a
caller writes N paths out and then reads N lines back in, it risks
becoming blocked on write() to check-*, and check-* is blocked on
write back to the caller.  Somebody has to buffer; the pipe buffers
provide some leeway, but they are limited.

Thanks to Peff for pointing this out:

    http://article.gmane.org/gmane.comp.version-control.git/220534

Signed-off-by: Adam Spiers <git@adamspiers.org>
t0008: remove duplicated test fixture data

The expected contents of STDOUT for the final --stdin tests can be
derived from the expected contents of STDOUT for the same tests when
--verbose is given, in the same way that test_expect_success_multi
derives this for earlier tests.

Signed-off-by: Adam Spiers <git@adamspiers.org>
check-ignore: add -n / --non-matching option

If `-n` or `--non-matching` are specified, non-matching pathnames will
also be output, in which case all fields in each output record except
for <pathname> will be empty.  This can be useful when running
check-ignore as a background process, so that files can be
incrementally streamed to STDIN, and for each of these files, STDOUT
will indicate whether that file matched a pattern or not.  (Without
this option, it would be impossible to tell whether the absence of
output for a given file meant that it didn't match any pattern, or
that the result simply hadn't been flushed to STDOUT yet.)

Signed-off-by: Adam Spiers <git@adamspiers.org>