Running test scripts under -x option of the shell is often not a
useful way to debug them, because the error messages from the
commands tests try to capture and inspect are contaminated by the
tracing output by the shell.  An earlier work done to make it more
pleasant to run tests under -x with recent versions of bash is
extended to cover posix shells that do not support BASH_XTRACEFD.

* sg/test-x:
  travis-ci: run tests with '-x' tracing
  t/README: add a note about don't saving stderr of compound commands
  t1510-repo-setup: mark as untraceable with '-x'
  t9903-bash-prompt: don't check the stderr of __git_ps1()
  t5570-git-daemon: don't check the stderr of a subshell
  t5526: use $TRASH_DIRECTORY to specify the path of GIT_TRACE log file
  t5500-fetch-pack: don't check the stderr of a subshell
  t3030-merge-recursive: don't check the stderr of a subshell
  t1507-rev-parse-upstream: don't check the stderr of a shell function
  t: add means to disable '-x' tracing for individual test scripts
  t: prevent '-x' tracing from interfering with test helpers' stderr