t3030-merge-recursive: don't check the stderr of a subshell

The two test checking 'git mmerge-recursive' in an empty worktree in 't3030-merge-recursive.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 emptiness of a subshell's stderr, which includes the trace of commands executed in that subshell as well, throwing off the emptiness check. Note that both subshells execute four git commands each, meaning that checking the emptiness of the whole subshell implicitly ensures that not only 'git merge-recursive' but none of the other three commands outputs anything to their stderr. Note also that if one of those commands were to output anything on its stderr, then the current combined check would not tell us which one of those four commands the unexpected output came from. Save the stderr of those four commands only instead of the whole subshell, so it remains free from tracing output, and save and check them individually, so they will show us from which command the unexpected output came from. After this change t3030 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>