IDEA-122626 Github: try to reuse existing access token

Github allows to create only one token with the same "Note"
(cherry picked from commit e8fa6ae)
diff: do not break layout if we don't know revision number
(cherry picked from commit e132e12)
IDEA-123286 diff: fix off-by-one in the current document too
(cherry picked from commit 6b4ad69)