test: facilitate debugging Git executables in tests with gdb

When prefixing a Git call in the test suite with 'debug ', it will
now be run with GDB, allowing the developer to debug test failures
more conveniently.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib-functions.sh

@@ -145,6 +145,14 @@ test_pause () {
 	fi
 }
 
+# Wrap git in gdb. Adding this to a command can make it easier to
+# understand what is going on in a failing test.
+#
+# Example: "debug git checkout master".
+debug () {
+	 GIT_TEST_GDB=1 "$@"
+}
+
 # Call test_commit with the arguments "<message> [<file> [<contents> [<tag>]]]"
 #
 # This will commit a file with the given contents and the given commit