t1507-rev-parse-upstream: don't check the stderr of a shell function
Three tests in 't1507-rev-parse-upstream.sh' fail when the test script
is run with '-x' tracing (and using a shell other than a Bash version
supporting BASH_XTRACEFD). The reason for those failures is that the
tests check the stderr of the function 'error_message', which includes
the trace of commands executed in that function as well, throwing off
the comparison with the expected output.
Save stderr of 'git rev-parse' only instead of the whole function, so
it remains free from tracing output.
After this change t1507 passes with '-x', even when running with
/bin/sh.
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>