and so checkout only the HEAD of the branch.
This leads to not being able to find an ancestor since there is only one
new commit. I presume that when it worked, it was because of specific
settings of some users.
Now we fetch (with the same depth as the target branch) the source branch
and use it to compare sha1 and find the common ancestor.
Closes#2292
Signed-off-by: Frederic Martinsons <frederic.martinsons@sigfox.com>
