Auto merge of #5110 - alexcrichton:reset-harder, r=matklad

Try a lot harder to recover corrupt git repos

We've received a lot of intermittent bug reports historically about corrupt git
repositories. These inevitably happens as Cargo is ctrl-c'd or for whatever
other reason, and to provide a better user experience Cargo strives to
automatically handle these situations by blowing away the old checkout for a new
update.

This commit adds a new test which attempts to pathologically corrupt a git
database and checkout in an attempt to expose bugs in Cargo. Sure enough there
were some more locations that we needed to handle gracefully for corrupt git
checkouts. Notable inclusions were:

* The `fetch` operation in libgit2 would fail due to corrupt references. This
  starts by adding an explicit whitelist for classes of errors coming out of
  `fetch` to auto-retry by blowing away the repository. We need to be super
  careful here as network errors commonly come out of this function and we don't
  want to too aggressively re-clone.

* After a `fetch` succeeded a repository could fail to actual resolve a
  git reference to the actual revision we want. This indicated that we indeed
  needed to blow everything away and re-clone entirely again.

* When creating a checkout from a database the `reset` operation might fail due
  to a corrupt local database of the checkout itself. If this happens we needed
  to just blow it away and try again.

There's likely more lurking situations where we need to re-clone but I figure we
can discover those over time.