[PATCH] test-lib: guard against being run as 'prove t*.sh'

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

We have t/Makefile targets to properly run the tests with prove,
however it's also just as valid to run them as e.g.:

    prove t[0-9]*.sh

But if they're run as:

    prove t*.sh

The prove tool will optimistically try to parse and execute
test-lib.sh and test-lib-functions.sh as Perl code, currently there
are no bad results of doing this other than a wall of error messages,
but let's be helpful and bail out early in this case.

Note that the shebang being added here doesn't mean anything to a
POSIX OS because the files aren't executable, it's picked up by Perl's
TAP tools. If the shebang wasn't here this would still be considered a
Perl file for the purposes of those heuristics.

I could also use the "Bail out!" keyword here, see
614fe01521 (test-lib: bail out when "-v" used under "prove",
2016-10-22) for prior art. I think it's more gentle to just print a
SKIP notice.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 t/test-lib-functions.sh | 9 +++++++++
 t/test-lib.sh           | 9 +++++++++
 2 files changed, 18 insertions(+)

diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 6348e8d733..7ba04fe399 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -1,3 +1,5 @@
+#!/bin/sh
+#
 # Library of functions shared by all tests scripts, included by
 # test-lib.sh.
 #
@@ -16,6 +18,13 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see http://www.gnu.org/licenses/ .
 
+# Early bailing out on 'prove t*.sh'
+if test -n "$HARNESS_ACTIVE" -a "$0" = "test-lib-functions.sh"
+then
+	echo "1..0 # SKIP accidentally invoked 'prove t*.sh'?"
+	exit 0
+fi
+
 # The semantics of the editor variables are that of invoking
 # sh -c "$EDITOR \"$@\"" files ...
 #
diff --git a/t/test-lib.sh b/t/test-lib.sh
index d3f6af6a65..7ceae9a38d 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -1,3 +1,5 @@
+#!/bin/sh
+#
 # Test framework for git.  See t/README for usage.
 #
 # Copyright (c) 2005 Junio C Hamano
@@ -15,6 +17,13 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see http://www.gnu.org/licenses/ .
 
+# Early bailing out on 'prove t*.sh'
+if test -n "$HARNESS_ACTIVE" -a "$0" = "test-lib.sh"
+then
+	echo "1..0 # SKIP accidentally invoked 'prove t*.sh'?"
+	exit 0
+fi
+
 # Test the binaries we have just built.  The tests are kept in
 # t/ subdirectory and are run in 'trash directory' subdirectory.
 if test -z "$TEST_DIRECTORY"
-- 
2.31.0.rc2.211.g1d0b8788b3