avoid needlessly connecting to unrelated tramp remotes

In `magit-save-buffers-predicate-tree-only' avoid needlessly connecting
to unrelated tramp remotes by first checking whether the absolute path
to the top-level of the current directory is a prefix of
`buffer-file-name'.  Only if that is so check whether `buffer-file-name'
is actually directly in that repository (it may also be part of a
submodule or other nested repository.

Fixes #1473.

Suggested-by: Ryan C. Thompson <rct@thompsonclan.org>