patch-tool-mxe: omit signature with git version

Signature is a string like:

    --
    1.9.1

Decrease patch diff size.

See #983

tools/patch-tool-mxe

@@ -94,7 +94,12 @@ function export_patch {
     echo ''
     echo 'Contains ad hoc patches for cross building.'
     echo ''
-    git format-patch --no-numbered -p --stdout dist..HEAD
+    git format-patch \
+        --no-numbered \
+        -p \
+        --no-signature \
+        --stdout \
+        dist..HEAD
   ) > $mxedir/src/${pkg}-${patch_name}.patch && \
   echo "Generated ${mxedir}/src/${pkg}-${patch_name}.patch"
 }