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>

Documentation/fetch-options.txt

@@ -135,7 +135,10 @@ ifndef::git-pull[]
 	specified refspec (can be given more than once) to map the
 	refs to remote-tracking branches, instead of the values of
 	`remote.*.fetch` configuration variables for the remote
-	repository.  See section on "Configured Remote-tracking
+	repository.  Providing an empty `<refspec>` to the
+	`--refmap` option causes Git to ignore the configured
+	refspecs and rely entirely on the refspecs supplied as
+	command-line arguments. See section on "Configured Remote-tracking
 	Branches" for details.
 
 -t::

t/t5510-fetch.sh

@@ -174,6 +174,30 @@ test_expect_success 'fetch --prune --tags with refspec prunes based on refspec'
 	git rev-parse sometag
 '
 
+test_expect_success '--refmap="" ignores configured refspec' '
+	cd "$TRASH_DIRECTORY" &&
+	git clone "$D" remote-refs &&
+	git -C remote-refs rev-parse remotes/origin/master >old &&
+	git -C remote-refs update-ref refs/remotes/origin/master master~1 &&
+	git -C remote-refs rev-parse remotes/origin/master >new &&
+	git -C remote-refs fetch --refmap= origin "+refs/heads/*:refs/hidden/origin/*" &&
+	git -C remote-refs rev-parse remotes/origin/master >actual &&
+	test_cmp new actual &&
+	git -C remote-refs fetch origin &&
+	git -C remote-refs rev-parse remotes/origin/master >actual &&
+	test_cmp old actual
+'
+
+test_expect_success '--refmap="" and --prune' '
+	git -C remote-refs update-ref refs/remotes/origin/foo/otherbranch master &&
+	git -C remote-refs update-ref refs/hidden/foo/otherbranch master &&
+	git -C remote-refs fetch --prune --refmap="" origin +refs/heads/*:refs/hidden/* &&
+	git -C remote-refs rev-parse remotes/origin/foo/otherbranch &&
+	test_must_fail git -C remote-refs rev-parse refs/hidden/foo/otherbranch &&
+	git -C remote-refs fetch --prune origin &&
+	test_must_fail git -C remote-refs rev-parse remotes/origin/foo/otherbranch
+'
+
 test_expect_success 'fetch tags when there is no tags' '
 
     cd "$D" &&