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

.travis_do.sh

@@ -146,6 +146,11 @@ echo_blue "=== Travis ENV"
 env
 echo_blue "=== Travis ENV"
 
+until git merge-base ${TRAVIS_COMMIT_RANGE/.../ } > /dev/null; do
+	echo_blue "Fetching 50 commits more"
+	git fetch origin --deepen=50
+done
+
 if [ "$TRAVIS_PULL_REQUEST" = false ] ; then
 	echo "Only Pull Requests are supported at the moment." >&2
 	exit 0