Re-enable style check.

Tell git to give us 20 lines of context, which hopefully is enough to
deal with some spurious warnings.

Change-Id: I97cb572f7b89ff305f46290d20ed0b4674af1f5b

.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 -U20 HEAD~40 | ./tools/scripts/checkpatch.pl --no-signoff -
     - ./bootstrap && ./configure --enable-remote-bitbang --enable-jtag_vpi $CONFIGURE_ARGS && make
     - file src/$EXECUTABLE