fetch: document and test --refmap=""

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 --refmap='' <remote> +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> Signed-off-by: Junio C Hamano <gitster@pobox.com>