riscv: attempt something else for travis (probably won't work otherwise Tim would have already done it this way...)

.travis.yml

@@ -55,6 +55,6 @@ script:
     # 50 changes any case. Most merges won't consist of more than 40 changes,
     # so this should work fine most of the time, and be a lot better than not
     # checking at all.
-    - git diff HEAD~40 | ./tools/scripts/checkpatch.pl --no-signoff -
+    - git diff origin/riscv | ./tools/scripts/checkpatch.pl --no-signoff -
     - ./bootstrap && ./configure --enable-remote-bitbang --enable-jtag_vpi $CONFIGURE_ARGS && make
     - file src/$EXECUTABLE