This change limits the number of commits we'll pull from Gitaly when
processing hooks for branch/tag push/delete actions to at-most 101.

This limit already exists when creating the initial default branch, but
when creating a new non-default branch, or pushing to an existing one,
we would read all the commits in the branch from Gitaly before throwing
all but the most recent 100 away and processing those for the hooks.

No behaviour changes are implied by this change, we're just skipping
unnecessary work by taking advantage of the new `limit` option to the
CommitsBetween RPC.

Since we never want the full list of `commits`, I change the interface
declared in `Git::BaseHooksService` to make it clearer as well.

Changelog: performance