Fix RevWalkUtils.findBranchesReachableFrom not finding some branches

Fix RevWalkUtils.findBranchesReachableFrom not finding some branches

The "cut off" optimization causes it to not include branches that
contain the specified commit but happen to share commits with a branch
that does not contain the commit.

An example:

      -B foo
        \
    -A---C master

findBranchesReachableFrom for commit A with both branches as input may
not return master (depending on the order of the input). The reason is
that A is not contained in foo, and therefore the old code would put B
in the cutOff set. When then walking the master commits and B is
checked, it is found in the cutOff set and the walk is aborted, causing
master not to be returned even though it should.

Bug: 425674
Change-Id: I2c0c406ce5fcc9a03538b483473af930d4895d30
Signed-off-by: Robin Stocker <robin@nibor.org>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>