Log Message:
Update to 3.28:

3.26: 2015-07-05
     Make repotool honor the mirror directory argument for CVS.
     Fix gitlab issue 4: even successful CVS export returns error code.
     Fix some name lookup issues from the cache needing invalidation.
     Implement and document that action stamps from author dates are preferred.
     New --filter option to mailbox_out for selective emission of headers.
     Fix gitlab issue #9: blank lines in svn:ignore misinterpreted.

3.27: 2015-06-22
     Per-repo-type preserves aren't clobbered when there's a lister.
     Git repository hooks are preserved.
     Read support for Monotone, not yet very well tested.