t/README: add a note about don't saving stderr of compound commands

Explain in 't/README' why it is a bad idea to redirect and verify the
stderr of compound commands, in the hope that future contributions
will follow this advice and the test suite will keep working with '-x'
tracing and /bin/sh.

While at it, since we can now run the test suite with '-x' without
needing a Bash version supporting BASH_XTRACEFD, remove the now
outdated caution note about non-Bash shells from the description of
the '-x' option.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/README

@@ -84,9 +84,7 @@ appropriately before running "make".
 
 -x::
 	Turn on shell tracing (i.e., `set -x`) during the tests
-	themselves. Implies `--verbose`. Note that in non-bash shells,
-	this can cause failures in some tests which redirect and test
-	the output of shell functions. Use with caution.
+	themselves. Implies `--verbose`.
 	Ignored in test scripts that set the variable 'test_untraceable'
 	to a non-empty value, unless it's run with a Bash version
 	supporting BASH_XTRACEFD, i.e. v4.1 or later.
@@ -455,6 +453,22 @@ Don't:
    causing the next test to start in an unexpected directory.  Do so
    inside a subshell if necessary.
 
+ - save and verify the standard error of compound commands, i.e. group
+   commands, subshells, and shell functions (except test helper
+   functions like 'test_must_fail') like this:
+
+     ( cd dir && git cmd ) 2>error &&
+     test_cmp expect error
+
+   When running the test with '-x' tracing, then the trace of commands
+   executed in the compound command will be included in standard error
+   as well, quite possibly throwing off the subsequent checks examining
+   the output.  Instead, save only the relevant git command's standard
+   error:
+
+     ( cd dir && git cmd 2>../error ) &&
+     test_cmp expect error
+
  - Break the TAP output
 
    The raw output from your test may be interpreted by a TAP harness. TAP