project: stop symlinking info dir under .git/

Unsharing this directory shouldn't be a problem.  The current repo code
treated it as a file, and while that's actually incorrect, files & dirs
are basically treated the same, so it's practically the same.

Let's enumerate each subpath since there aren't that many.

info/refs:
Only used when the project is exported over git dumb transports (i.e.
a http:// server).  Repo never does this, and it's extremely unlikely
any user has ever done this.  Plus, this proposal talks about unsharing
project refs, so this file should get unshared too.

info/grafts:
A user-configurable file that repo never touches.  Might be useful to
share across projects, but probably rarely (if ever) used by developers,
and forcing them to configure it for each project isn't that big of a
deal.

info/exclude:
info/attributes:
User-configurable files that repo never touches.  Doesn't seem like
most users ever touch these, and if they do, having them do it for
each shared project isn't a big deal.

info/sparse-checkout:
Repo doesn't use sparse checkouts, and it's extremely unlikely to even
work if a user tried doing something themselves.

Bug: https://crbug.com/gerrit/15460
Change-Id: I53e44d73a6d7a92da615b46600d8ea51cb46e3ac
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/327519
Reviewed-by: Jonathan Nieder <jrn@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

project.py

@@ -457,7 +457,7 @@ class RemoteSpec(object):
 
 class Project(object):
   # These objects can be shared between several working trees.
-  shareable_files = ['info']
+  shareable_files = []
   shareable_dirs = ['hooks', 'objects', 'rr-cache']
 
   def __init__(self,