Enable GPG in the Windows part of the CI/PR builds

While debugging the breakages introduced by hi/gpg-prefer-check-signature, I noticed that the GPG prereq was not available on Windows, even if Git for Windows' SDK comes with a fully functional GPG2.

The fix was easy, but finding out what was going on was not, so for good measure, the fix is accompanied by a patch that will hopefully make future investigations into GPG-related problems much, much easier.

Changes since v2:

* Reordered 4/5 before 3/5, as I had intended originally.

* Renamed _trace_level to have a trailing underscore, in line with the surrounding code.

* Added a note to the commit message why only lib-gpg.sh loses its hash-bang line, and no other files in t/.

Changes since v1:

* The prereqs are now lazy ones.

* A new patch was introduced to make tracing via -x work even with those inter-dependent prereqs.

* The test-signing's stdout is redirected to /dev/null because it is unreadable and unhelpful binary gibberish, anyway. (This imitates Peff's patch.)

Johannes Schindelin (5):
tests(gpg): allow the gpg-agent to start on Windows t/lib-gpg.sh: stop pretending to be a stand-alone script
tests: do not let lazy prereqs inside test_expect_* turn off tracing
tests: turn GPG, GPGSM and RFC1991 into lazy prereqs
tests: increase the verbosity of the GPG-related prereqs

 t/lib-gpg.sh     | 110 ++++++++++++++++++++++++++---------------------
 t/t0000-basic.sh |  13 ++++++
 t/test-lib.sh    |   6 ++-
 3 files changed, 77 insertions(+), 52 deletions(-)

base-commit: 30e9940356dc67959877f4b2417da33ebdefbb79
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-728%2Fdscho%2Fci-windows-gpg-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-728/dscho/ci-windows-gpg-v3
Pull-Request: https://github.com/git/git/pull/728

Range-diff vs v2:

 1:  287a21f1033 = 1:  287a21f1033 tests(gpg): allow the gpg-agent to start on Windows
 2:  c1811d54190 ! 2:  b4217c36070 t/lib-gpg.sh: stop pretending to be a stand-alone script

@@ -5,6 +5,10 @@
It makes no sense to call ./lib-gpg.sh. Therefore the hash-bang line is unnecessary.

+ There are other similar instances in t/, but they are too far from the
+ context of the enclosing patch series, so they will be addressed
+ separately.
+

diff --git a/t/lib-gpg.sh b/t/lib-gpg.sh
4: 0767c8b77c8 ! 3: f35830c0eba tests: do not let lazy prereqs inside test_expect_* turn off tracing

     @@ -60,7 +60,7 @@
       	fi
       }
       
     -+_trace_level=0
     ++trace_level_=0
       want_trace () {
       	test "$trace" = t && {
       		test "$verbose" = t || test "$verbose_log" = t
     @@ -69,7 +69,7 @@
       	# Do not add anything extra (including LF) after '$*'
       	eval "
      -		want_trace && set -x
     -+		want_trace && _trace_level=$(($_trace_level+1)) && set -x
     ++		want_trace && trace_level_=$(($trace_level_+1)) && set -x
       		$*"
       }
       
     @@ -78,8 +78,8 @@
       		if want_trace
       		then
      -			set +x
     -+			test 1 = $_trace_level && set +x
     -+			_trace_level=$(($_trace_level-1))
     ++			test 1 = $trace_level_ && set +x
     ++			trace_level_=$(($trace_level_-1))
       		fi
       	} 2>/dev/null 4>&2

 3:  85457a7b618 = 4:  f69f97e24ba tests: turn GPG, GPGSM and RFC1991 into lazy prereqs
 5:  5e89b512513 = 5:  064f4e541b8 tests: increase the verbosity of the GPG-related prereqs

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