pathspec: die on empty strings as pathspec

pathspec: die on empty strings as pathspec

An empty string as a pathspec element matches all paths.  A buggy
script, however, could accidentally assign an empty string to a
variable that then gets passed to a Git command invocation, e.g.:

  path=... compute a path to be removed in $path ...
        git rm -r "$path"

which would unintentionally remove all paths in the current
directory.

The fix for this issue comprises of two steps. Step 1, which warns
that empty strings as pathspecs will become invalid, has already
been implemented in commit d426430 ("pathspec: warn on empty strings
as pathspec", 2016-06-22).

This patch is step 2. It removes the warning and throws an error
instead.

Signed-off-by: Emily Xie <emilyxxie@gmail.com>
Reported-by: David Turner <novalis@novalis.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>