t0091-bugreport.sh: actually verify some content of report

t0091-bugreport.sh: actually verify some content of report

In the first test in this script, 'creates a report with content in the
right places', we generate a report and pipe it into our helper
`check_all_headers_populated()`. The idea of the helper is to find all
lines that look like headers ("[Some Header Here]") and to check that
the next line is non-empty. This is supposed to catch erroneous outputs
such as the following:

  [A Header]
  something
  more here

  [Another Header]

  [Too Early Header]
  contents

However, we provide the lines of the bug report as filenames to grep,
meaning we mostly end up spewing errors:

  grep: : No such file or directory
  grep: [System Info]: No such file or directory
  grep: git version:: No such file or directory
  grep: git version 2.41.0.2.gfb7d80edca: No such file or directory

This doesn't disturb the test, which tugs along and reports success, not
really having verified the contents of the report at all.

Note that after 788a776069 ("bugreport: collect list of populated
hooks", 2020-05-07), the bug report, which is created in our hook-less
test repo, contains an empty section with the enabled hooks. Thus, even
the intention of our helper is a bit misguided: there is nothing
inherently wrong with having an empty section in the bug report.

Let's instead split this test into three: first verify that we generate
a report at all, then check that the introductory blurb looks the way it
should, then verify that the "[System Info]" seems to contain the right
things. (The "[Enabled Hooks]" section is tested later in the script.)

Reported-by: SZEDER Gábor <szeder.dev@gmail.com>
Helped-by: Phillip Wood <phillip.wood123@gmail.com>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>