test-lib: add the test_pause convenience function

Since 781f76b1 (test-lib: redirect stdin of tests) you can't simply put a
"bash &&" into a test for debugging purposes anymore. Instead you'll have
to use "bash <&6 >&3 2>&4".

As that invocation is not that easy to remember add the test_pause
convenience function. It invokes "$SHELL_PATH" to provide a sane shell
for the user.

This function also checks if the -v flag is given and will error out if
that is not the case instead of letting the test hang until ^D is pressed.
Signed-off-by: NJens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/README

@@ -548,6 +548,19 @@ library for your script to use.
 		...
 	'
 
+ - test_pause
+
+	This command is useful for writing and debugging tests and must be
+	removed before submitting. It halts the execution of the test and
+	spawns a shell in the trash directory. Exit the shell to continue
+	the test. Example:
+
+	test_expect_success 'test' '
+		git do-something >actual &&
+		test_pause &&
+		test_cmp expected actual
+	'
+
 Prerequisites
 -------------
 

t/test-lib.sh

@@ -329,6 +329,19 @@ test_tick () {
 	export GIT_COMMITTER_DATE GIT_AUTHOR_DATE
 }
 
+# Stop execution and start a shell. This is useful for debugging tests and
+# only makes sense together with "-v".
+#
+# Be sure to remove all invocations of this command before submitting.
+
+test_pause () {
+	if test "$verbose" = t; then
+		"$SHELL_PATH" <&6 >&3 2>&4
+	else
+		error >&5 "test_pause requires --verbose"
+	fi
+}
+
 # Call test_commit with the arguments "<message> [<file> [<contents>]]"
 #
 # This will commit a file with the given contents and the given commit