get rid of the revisionTable variable

[makohoek]

          It looks weird to return `branch` as such which is in the function parameters

I wish we could return a tuple like (remote/project/branch, revision), remote/project/branch being a key kind of... it'd make things easier to update revisions in update_manifest, avoiding the for url, branch, rev in revisionList: in update_manifest

but... a branch can contain '/' (slash) character and project name too, so it seems...
So the following 2 would match:
project "a/1" branch "alpha"
project "a" branch "1/alpha"

So that's the right thing to do: get rid of the revisionTable variable. Maybe we could store as revisionTable[projectRemote][project][branch] or something similar in a future release...?

Originally posted by @david-baylibre in #40 (comment)