Checkout a specific dev version of module using git
This is more a git question than Drupal, but you can use rev-list command to retrieve a list of commit hashes since the 7.x-2.0-alpha8 tag. Using the tail command you can reverse the list to show the first 100 commits (I think the +XX- scheme is an offset from the first commit) then filter that down to the last result (which should be +99-dev commit).

git rev-list 7.x-2.0-alpha8^..HEAD  | tail -r -n 100 | tail -n -1