travis: Report commit that is being built

Although the Travis user interface displays the hash it thinks it is
building, I'm not convinced that the test machine itself is guaranteed
to be building the right one. Let's emit the full log message for the
HEAD commit before starting the build so we can be sure.

.travis.yml

@@ -8,6 +8,7 @@ addons:
     packages: ninja-build libboost-dev libboost-system-dev libboost-thread-dev libboost-chrono-dev
 
 script:
+  - git log -1
   - mkdir -p travis-build
   - cd travis-build
   - ccache -s