git-svn: use `svn:global-ignores` to create .gitignore

git-svn: use `svn:global-ignores` to create .gitignore

`svn:global-ignores` contains a list of file patterns that should
not be tracked in version control. The syntax of these patterns is
the same as `svn:ignore`. Their semantics differ: patterns in
`svn:global-ignores` apply to all paths under the directory where
they apply, while `svn:ignore` only applies to the directory's
immediate children.

Signed-off-by: Alex Galvin <agalvin@comqi.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>