test-lib.sh: support -x option for shell-tracing

Usually running a test under "-v" makes it clear which command is failing. However, sometimes it can be useful to also see a complete trace of the shell commands being run in the test. You can do so without any support from the test suite by running "sh -x tXXXX-foo.sh". However, this produces quite a large bit of output, as we see a trace of the entire test suite. This patch instead introduces a "-x" option to the test scripts (i.e., "./tXXXX-foo.sh -x"). When enabled, this turns on "set -x" only for the tests themselves. This can still be a bit verbose, but should keep things to a more manageable level. You can even use "--verbose-only" to see the trace only for a specific test. The implementation is a little invasive. We turn on the "set -x" inside the "eval" of the test code. This lets the eval itself avoid being reported in the trace (which would be long, and redundant with the verbose listing we already showed). And then after the eval runs, we do some trickery with stderr to avoid showing the "set +x" to the user. We also show traces for test_cleanup functions (since they can impact the test outcome, too). However, we do avoid running the noop ":" cleanup (the default if the test does not use test_cleanup at all), as it creates unnecessary noise in the "set -x" output. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>