t7401: Tell diff to ignore whitespace on MinGW

"git submodule" prints CRLF lineendings, which break the comparison with
expected outputs.  This commit sets the diff command to ignore
whitespace changes.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>

t/t7401-submodule-summary.sh

@@ -8,6 +8,10 @@ test_description='Summary support for submodules
 This test tries to verify the sanity of summary subcommand of git-submodule.
 '
 
+case $(uname -s) in
+*MINGW*) GIT_TEST_CMP="diff -uw";;
+esac
+
 . ./test-lib.sh
 
 add_file () {