worktree support: dgit: Fix for .git/info/attributes

worktree support: dgit: Fix for .git/info/attributes

Experimentally, this file is in the common dir, not the per-worktree
dir.  Honour $maindir_gitcommon everywhere.

Rename `open_main_gitattrs'.  Previously it could in theory be used in
the playtree (but, it isn't).  Now it always operates on the main
tree.

Indeed, as we require, the `setup' stuff is only done in the main
tree.  The gitattributes fixes for playtrees are done,
unconditionally, with the much simpler hammer in playtree_setup.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>