Restore cache functionality

[vifactor]

Store fetched data in a easily readable format and next time when repo is about to be analyzed fetch only updated git objects.

[jonespm]

Our repository is large and takes almost 5 minutes to fetch the author info. Caching even that would be a nice improvement!

[vifactor]

yeah, I filed this issue when I tried to work on a repo where analysis phase lasted more than 20 mins. I have it in plans, but the feature is not something quick. Do not have a deadline, but definitely will prioritize.

[vifactor]

History cache might be organized as follows:

merge_base = pygit2.Repository.merge_base("cache head", "current head")

• if "cache head" exists in repo, one needs to use all commits "before" "merge_base" from cache and fetch data from
  "merge_base" to a current "head commit"

• if "cache head" is not in repo anymore:
  a) either fetch whole history from scratch
  b) or find a commit in cache that still exists in repo and find "merge_base" with "current_head"