[PATCH v2 6/6] ci: fix AsciiDoc/Asciidoctor stderr check in the documentation build job

In our 'ci/*' build scripts we rely on 'set -e' aborting the build job when a command exits with error, while in 'ci/test-documentation.sh' we tried to check the emptiness of AsciiDoc's and Asciidoctor's standard error with '! test -s stderr.log'. Unfortunately, the combination of the two doesn't work as intended, because, according to POSIX [1]:

  "The -e setting shall be ignored when executing [...] a pipeline
  beginning with the ! reserved word" [2]

Watch and learn:

  $ cat e.sh
  #!/bin/sh
  set -ex
  echo unexpected >file
  ! test -s file
  test ! -s file
  echo "should not reach this"
  $ ./e.sh
  + echo unexpected
  + test -s file
  + test ! -s file
  $

This is why we haven't noticed the warnings from Asciidoctor that were fixed in the first patches of this patch series, though some of them were already there in the build of v2.18.0-rc0 [3].

Check the emptiness of that file with 'test ! -s' instead, which works properly with 'set -e'. Furthermore, dump the contents of that file to the log for our convenience, so if it were to unexpectedly end up being non-empty, then we wouldn't have to scroll through all that long build log looking for warnings, but could see them right away near the end of the log.

And one more thing: we build the docs with Asciidoctor right after a 'make clean', meaning that 'make doc' starts with running 'GIT-VERSION-GEN', which in turn prints the version to its standard error. This version then goes to 'stderr.log' as well, and a 'sed' command is supposed to remove it to prevent it from interfering with that (previously defunct) emptiness check. Alas, this command doesn't work as intended, either, because it leaves the file to be checked intact, but the defunct emptiness check hid this issue, too... This patch deals with this as well.

[1] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#set

[2] POSIX doesn't discuss the meaning of '! cmd' in case of simple
    commands, but it defines that "A pipeline is a sequence of one or
    more commands separated by the control operator '|'", so
    apparently a simple command is considered as pipeline as well.

    http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_02

[3] https://travis-ci.org/git/git/jobs/385932007#L1463

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 ci/test-documentation.sh | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)