Make running git under other debugger-like programs easy

This allows us to run git, when using the script from bin-wrappers, under
other programs.  A few examples for usage within testsuite scripts:

   debug git checkout master
   debug --debugger=nemiver git $ARGS
   debug -d "valgrind --tool-memcheck --track-origins=yes" git $ARGS

Or, if someone has bin-wrappers/ in their $PATH and is executing git
outside the testsuite:

   GIT_DEBUGGER="gdb --args" git $ARGS
   GIT_DEBUGGER=nemiver git $ARGS
   GIT_DEBUGGER="valgrind --tool=memcheck --track-origins=yes" git $ARGS

There is also a handy shortcut of GIT_DEBUGGER=1 meaning the same as
GIT_DEBUGGER="gdb --args"

Original-patch-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib-functions.sh

@@ -145,12 +145,28 @@ test_pause () {
 	"$SHELL_PATH" <&6 >&5 2>&7
 }
 
-# Wrap git in gdb. Adding this to a command can make it easier to
-# understand what is going on in a failing test.
+# Wrap git with a debugger. Adding this to a command can make it easier
+# to understand what is going on in a failing test.
 #
-# Example: "debug git checkout master".
+# Examples:
+#     debug git checkout master
+#     debug --debugger=nemiver git $ARGS
+#     debug -d "valgrind --tool=memcheck --track-origins=yes" git $ARGS
 debug () {
-	 GIT_TEST_GDB=1 "$@" <&6 >&5 2>&7
+	case "$1" in
+	-d)
+		GIT_DEBUGGER="$2" &&
+		shift 2
+		;;
+	--debugger=*)
+		GIT_DEBUGGER="${1#*=}" &&
+		shift 1
+		;;
+	*)
+		GIT_DEBUGGER=1
+		;;
+	esac &&
+	GIT_DEBUGGER="${GIT_DEBUGGER}" "$@" <&6 >&5 2>&7
 }
 
 # Call test_commit with the arguments