t5536: simplify checking of messages output to stderr

t5536: simplify checking of messages output to stderr

Commit 2071e05ed2 ("t5536: new test of refspec conflicts when
fetching", 2013-10-30), introduced the verify_stderr() function
which was used to verify that certain fatal/warning messages were
issued by a given git command. In addition, verify_stderr() would
filter a specific "fatal: The remote end hung up unexpectedly"
message, which may, or may not, be present (depending on the
relative timing of the git-fetch and git-upload-pack processes).

The verify_stderr() function has seen several modifications, which
has introduced a couple of minor problems. For example, commit
1edbaac3bb ("tests: use test_i18n* functions to suppress false
positives", 2016-06-17) introduced an inappropriate test_i18ngrep
call and commit f096e6e826 ("fetch: improve the error messages
emitted for conflicting refspecs", 2013-10-30) included an
ineffective invocation of sort at the end of a grep pipeline.

Instead of fixing these minor problems in verify_stderr(), we take
the simpler approach of directly searching the error file, using
test_i18ngrep, for the specific message(s) we expect. (The only
minor downside is that we would not notice any new messages).

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>