GHSA-555P-6GRF-MH7F Dulwich doesn't sanitize commit subjects in `porcelain.format_patch`

Impact dulwich.porcelain.formatpatchoutdir=... derives each patch filename from the commit's subject line. Prior to this fix, getsummary only replaced spaces with dashes - path separators /, , parent-directory components .., and other filename-hostile characters e.g. : were preserved verbatim and...