bug#16897: 24.3.50; vc-print-root-log doesn't show the top commit if it's a merge commit

[Dmitry Gutov <dgutov@yandex.ru>]

1. Open a Git repo.

2. Do a simple, non-fast-forward merge from some branch into the current
(but it has to succeed without conflicts).

3. Press `C-x v L`.

The top commit will be hidden, apparently because 'git log' is passed
the 'path' argument, namely the root directory of the repository, and so
it somehow ignores the commit that doesn't add anything to the histroy
beyond the contents of its parents.

Using Git 1.8.3.2.

If I replace `files' with 'nil' in `vc-git-print-log', the top commit is
displayed. Not sure how to handle this best. Perhaps just compare
`files' with (list default-directory)?

That's assuming at least some other backend needs a non-nil valus of
`files' to retrieve the root log.


In GNU Emacs 24.3.50.3 (x86_64-unknown-linux-gnu, GTK+ Version 3.8.6)
 of 2014-02-20 on axl
Repository revision: 116484 lekktu@gmail.com-20140219210406-y2s7lx244ojfl5on
Windowing system distributor `The X.Org Foundation', version 11.0.11405000
System Description:	Ubuntu 13.10