test-lib: check Bash version for '-x' without using shell arrays

One of our test scripts, 't1510-repo-setup.sh' [1], still can't be
reliably run with '-x' tracing enabled, unless it's executed with a
Bash version supporting BASH_XTRACEFD (since v4.1).  We have a lengthy
condition to check the version of the shell running the test script,
and disable tracing if it's not executed with a suitable Bash version
[2].

This condition uses non-portable shell array accesses to easily get
Bash's major and minor version number.  This didn't seem to be
problematic, because the simple commands expanding those array
accesses are only executed when the test script is actually run with
Bash.  When run with Dash, the only shell I have at hand that doesn't
support shell arrays, there are no issues, as it apparently skips
right over the non-executed simple commands without noticing the
non-supported constructs.

Alas, it has been reported that NetBSD's /bin/sh does complain about
them:

  ./test-lib.sh: 327: Syntax error: Bad substitution

where line 327 contains the first ${BASH_VERSINFO[0]} array access.

To my understanding both shells are right and conform to POSIX,
because the standard allows both behaviors by stating the following
under '2.8.1 Consequences of Shell Errors' [3]:

  "An expansion error is one that occurs when the shell expansions
  define in wordexp are carried out (for example, "${x!y}", because
  '!' is not a valid operator); an implementation may treat these as
  syntax errors if it is able to detect them during tokenization,
  rather than during expansion."

Avoid this issue with NetBSD's /bin/sh (and potentially with other,
less common shells) by hiding the shell array syntax behind 'eval'
that is only executed with Bash.

[1] 5827506928 (t1510-repo-setup: mark as untraceable with '-x',
    2018-02-24)
[2] 5fc98e79fc (t: add means to disable '-x' tracing for individual
    test scripts, 2018-02-24)
[3] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_08_01

Reported-by: Max Kirillov <max@max630.net>
Helped-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -270,12 +270,12 @@ do
 		# this test is marked as such, and ignore '-x' if it
 		# isn't executed with a suitable Bash version.
 		if test -z "$test_untraceable" || {
-		     test -n "$BASH_VERSION" && {
+		     test -n "$BASH_VERSION" && eval '
 		       test ${BASH_VERSINFO[0]} -gt 4 || {
 			 test ${BASH_VERSINFO[0]} -eq 4 &&
 			 test ${BASH_VERSINFO[1]} -ge 1
 		       }
-		     }
+		     '
 		   }
 		then
 			trace=t