Stop using 'git checkout' in pre_receive.

As of earlier this year, git no longer allows any ref updates in the
pre-receive hook.

This is intended to prevent corruption of your git tree, but it means
that we can't actually do a git checkout in pre-receive.

(See https://www.spinics.net/lists/git/msg300763.html for the git side
details.)

Instead, we are using git archive to dump the tree as of the new commit,
and we are decompressing that in our new temporary directory
(commit 6bbfc5f).

We then chdir to the temporary directory, do all of our checks, and
switch back to the repo directory before saving the list file.

bin/gitzone

@@ -312,8 +312,13 @@ sub pre_receive {
     File::Path->remove_tree($tmp_dir, verbose => 1);
     File::Path->make_path($tmp_dir, verbose => 1);
 
-    # checkout changes
-    git "checkout -qf $new";
+    # Extract the new commit.
+    # We do this with git archive, and then extract the resulting tar in the temporary directory.
+    # There really should be a better way to do this, but I can't find one.
+    git "archive $new | tar -C $tmp_dir -xf -";
+
+    # chdir into the temporary directory.
+    chdir $tmp_dir or die $!;
 
     # Go read only, no actual changes in the pre-release hook.
     $read_only = 1;
@@ -322,6 +327,8 @@ sub pre_receive {
     load_repo_config;
     process_files;
 
+    # Go back to the repo.
+    chdir $base_cwd;
 
     # Save the file list, this is of questionable value at this point.
     save_list_file;