git-svn: mention `svn:global-ignores` in help+docs

git-svn: mention `svn:global-ignores` in help+docs

Git-SVN was previously taught to use the svn:global-ignores property as
well as svn:ignore when creating or showing .gitignore files from a
Subversion repository. However, the documentation and help message still
only mentioned svn:ignore. Update Git-SVN's documentation and help
command to mention support for the new property. Also capitalize the
help message for the 'mkdirs' command, for consistency.

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