Ignore .gitattributes in bare repositories

Ignore .gitattributes in bare repositories

Attributes can be specified at three different places: the internal
table of default values, the file $GIT_DIR/info/attributes and files
named .gitattributes in the work tree.  Since bare repositories don't
have a work tree, git should ignore any .gitattributes files there.

This patch makes git do that, so the only way left for a user to specify
attributes in a bare repository is the file info/attributes (in addition
to changing the defaults and recompiling).

In addition, git-check-attr is now allowed to run without a work tree.
Like any user of the code in attr.c, it ignores the .gitattributes files
when run in a bare repository.  It can still read from info/attributes.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>