Better handling big history by fetching changesets by lots

of 100 max (and creating commits in the same time)
before fetching another lot if needed....

& little refactoring
Merge pull request #626 from pmiossec/fetch_by_lots

[Need Tests] Fetch by lots
Use libgit2sharp to find commits to checkin
Adapt FindParentCommits() to return nothing when commits are not parents
Unit test