t1507-rev-parse-upstream: don't check the stderr of a shell function

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>