Arguments starting with '@@' are no paths

... so let's skip posix-to-windows conversion on such parameters.

This fixes Git's t1508-at-combinations.sh.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

winsup/cygwin/msys2_path_conv.cc

@@ -381,6 +381,11 @@ path_type find_path_start_and_type(const char** src, int recurse, const char* en
                     goto skip_p2w;
             }
             break;
+        case '@':
+            // Paths do not contain '@@'
+            if (it + 1 < end && it[1] == '@')
+                goto skip_p2w;
+        }
         ++it;
     }
     it = *src;