Since MergeRequestDiff is not about branches and current state of merge
request diff anymore I removed most of branch related method and added
validation for head/start/base commit sha. From this point
MergeRequestDiff is about saving diff between branches only once at moment of
creation. Once created MergeRequestDiff should not be changes. Because
of that we should not rely on changes in source/target branches when
read from MergeRequestDiff

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>