reset: improve mixed reset error message when in a bare repo

reset: improve mixed reset error message when in a bare repo

When running a "git reset --mixed" in a bare repository, the
message displayed is something like:

fatal: This operation must be run in a work tree
fatal: Could not reset index file to revision 'HEAD^'.

This message is a little bit misleading because a mixed reset is
ok in a git directory, so it is not absolutely needed to run it in
a work tree.

So this patch improves upon the above by changing the message to:

fatal: mixed reset is not allowed in a bare repository

And if "git reset" is ever sped up by using unpack_tree() directly
(instead of execing "git read-tree"), this patch will also make
sure that a mixed reset is still disallowed in a bare repository.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>