gitignore: root most patterns at the top-level directory

gitignore: root most patterns at the top-level directory

Our gitignore doesn't use a preceding "/" to root its
patterns in the top of the repository. This means that if
you add a file or directory called "git" (for example)
inside a subdirectory, it will be erroneously ignored.

This patch was done mechanically with "s/^[^*]/\/&/" with
one exception: instead of ignoring gitk-wish, we should
gitk-git/gitk-wish (arguably, this should be done in
gitk-git/.gitignore, but because that is a subtree merge
from elsewhere, this is easier).

Acked-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>