Merge branch 'db/length-as-hash' (early part) into db/svn-fe-code-purge

Merge branch 'db/length-as-hash' (early part) into db/svn-fe-code-purge

* 'db/length-as-hash' (early part):
  vcs-svn: implement perfect hash for top-level keys
  vcs-svn: implement perfect hash for node-prop keys
  vcs-svn: improve reporting of input errors
  vcs-svn: make buffer_copy_bytes return length read
  vcs-svn: make buffer_skip_bytes return length read
  vcs-svn: improve support for reading large files

Conflicts:
	vcs-svn/fast_export.c
	vcs-svn/svndump.c