Make !pattern in .gitattributes non-fatal

Before 82dce99 (attr: more matching optimizations from .gitignore,
2012-10-15), .gitattributes did not have any special treatment of a
leading '!'. The docs, however, always said

The rules how the pattern matches paths are the same as in
`.gitignore` files; see linkgit:gitignore[5].

By those rules, leading '!' means pattern negation. So 82dce99
correctly determined that this kind of line makes no sense and should
be disallowed.

However, users who actually had a rule for files starting with a '!'
are in a bad position: before 82dce99 '!' matched that literal
character, so it is conceivable that users have .gitattributes with
such lines in them. After 82dce99 the unescaped version was
disallowed in such a way that git outright refuses to run(!) most
commands in the presence of such a .gitattributes. It therefore
becomes very hard to fix, let alone work with, such repositories.

Let's at least allow the users to fix their repos: change the fatal
error into a warning.

Reported-by: mathstuf@gmail.com
Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>