Re: [PATCH v2] fetch: document and test --refmap=""

> From: Derrick Stolee <dstolee@microsoft.com>
>
> To prevent long blocking time during a 'git fetch' call, a user
> may want to set up a schedule for background 'git fetch' processes.
> However, these runs will update the refs/remotes branches due to
> the default refspec set in the config when Git adds a remote.
> Hence the user will not notice when remote refs are updated during
> their foreground fetches. In fact, they may _want_ those refs to
> stay put so they can work with the refs from their last foreground
> fetch call.
>
> This can be accomplished by overriding the configured refspec using
> '--refmap=' along with a custom refspec:
>
>   git fetch <remote> --refmap= +refs/heads/*:refs/hidden/<remote>/*
>
> to populate a custom ref space and download a pack of the new
> reachable objects. This kind of call allows a few things to happen:
>
> 1. We download a new pack if refs have updated.
> 2. Since the refs/hidden branches exist, GC will not remove the
>    newly-downloaded data.
> 3. With fetch.writeCommitGraph enabled, the refs/hidden refs are
>    used to update the commit-graph file.
>
> To avoid the refs/hidden directory from filling without bound, the
> --prune option can be included. When providing a refspec like this,
> the --prune option does not delete remote refs and instead only
> deletes refs in the target refspace.
>
> Update the documentation to clarify how '--refmap=""' works and
> create tests to guarantee this behavior remains in the future.
>
> Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
> ---