share: replace the bookmarks.shared file with an entry on a new "shared" file

cd79fb4d75fd introduced a way to share bookmarks. When a repository share that
shares bookmarks was created, a .hg/bookmarks.shared file was created to mark
the repository share as one that shares its bookmarks.

We have plans to introduce other levels of sharing, including a "full share"
mode. Rather than creating a new ".shared" file for each new thing that we may
want to share It seems better to create a single "shared" file that will list
what is shared for a given shared repository. This should make it much easier
to get a list of everything that is shared by a given shared repository.

The shared file contains a list of shared "items" (such as bookmarks). Each
shared "item" is added as a new line in the file. For now the only possible
entry in the file is "bookmarks".

hgext/share.py

@@ -79,12 +79,12 b' def extsetup(ui):'
 def _hassharedbookmarks(repo):
     """Returns whether this repo has shared bookmarks"""
     try:
-        repo.vfs.read('bookmarks.shared')
-        return True
+        shared = repo.vfs.read('shared').splitlines()
     except IOError, inst:
         if inst.errno != errno.ENOENT:
             raise
         return False
+    return 'bookmarks' in shared
 
 def _getsrcrepo(repo):
     """

mercurial/hg.py

@@ -226,7 +226,9 b' def share(ui, source, dest=None, update='
         _update(r, uprev)
 
     if bookmarks:
-        r.vfs('bookmarks.shared', 'w').close()
+        fp = r.vfs('shared', 'w')
+        fp.write('bookmarks\n')
+        fp.close()
 
 def copystore(ui, srcrepo, destpath):
     '''copy files from store of srcrepo in destpath