Use the branch icon for indicating stable plugin branches

Use a symbol that is more similar to a branch for indicating
that a plugin has a stable branch associated to the corresponding
Gerrit one.

Previously a black check did not have any association with the
concept of "branch" and could have been confusing with the
green check mark.

Change-Id: Id6638b036ac270783d07155ef3286f5ded8380a9
2 files changed