Travis: fix bench fetch in case of PR (#968)

When Travis tests a PR, a commit merge is created
but master branch is not updated, although HEAD is.

No functional change.

.travis.yml

@@ -48,7 +48,7 @@ script:
   # checking bench for various build types
   #
   # obtain reference from git log
-  - git log | grep "\b[Bb]ench[ :]\+[0-9]\{7\}" | head -n 1 | sed "s/[^0-9]*\([0-9][0-9]*\)/\1/g" > git_sig
+  - git log HEAD | grep "\b[Bb]ench[ :]\+[0-9]\{7\}" | head -n 1 | sed "s/[^0-9]*\([0-9][0-9]*\)/\1/g" > git_sig
   - export benchref=$(cat git_sig)
   - echo "Reference bench:" $benchref
   # verify against reference