build,travis: fetch commits until we find merge-base

if the merge-base is not in the initial clone,
git diff / git rev-list will return funky results

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>

.travis.yml

@@ -1,5 +1,5 @@
 git:
-  depth: 1
+  depth: 10
 language: c
 dist: trusty
 sudo: false