cache_ref_iterator_begin(): avoid priming unneeded directories

When iterating over references, reference priming is used to make sure that loose references are read into the ref-cache before packed references, to avoid races. It used to be that the prefix passed to reference iterators almost always ended in `/`, for example `refs/heads/`. In that case, the priming code would read all loose references under `find_containing_dir("refs/heads/")`, which is "refs/heads/". That's just what we want. But now that `ref-filter` knows how to pass refname prefixes to `for_each_fullref_in()`, the prefix might come from user input; for example, git for-each-ref refs/heads Since the argument doesn't include a trailing slash, the reference iteration code would prime all of the loose references under `find_containing_dir("refs/heads")`, which is "refs/". Thus we would unnecessarily read tags, remote-tracking references, etc., when the user is only interested in branches. It is a bit awkward to get around this problem. We can't just append a slash to the argument, because we don't know ab initio whether an argument like `refs/tags/release` corresponds to a single tag or to a directory containing tags. Moreover, until now a `prefix_ref_iterator` was used to make the final decision about which references fall within the prefix (the `cache_ref_iterator` only did a rough cut). This is also inefficient, because the `prefix_ref_iterator` can't know, for example, that while you are in a subdirectory that is completely within the prefix, you don't have to do the prefix check. So: * Move the responsibility for doing the prefix check directly to `cache_ref_iterator`. This means that `cache_ref_iterator_begin()` never has to wrap its return value in a `prefix_ref_iterator`. * Teach `cache_ref_iterator_begin()` (and `prime_ref_dir()`) to be stricter about what they iterate over and what directories they prime. * Teach `cache_ref_iterator` to keep track of whether the current `cache_ref_iterator_level` is fully within the prefix. If so, skip the prefix checks entirely. The main benefit of these optimizations is for loose references, since packed references are always read all at once. Note that after this change, `prefix_ref_iterator` is only ever used for its trimming feature and not for its "prefix" feature. But I'm not ripping out the latter yet, because it might be useful for another patch series that I'm working on. Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>