convert: add copyright and license headers to back-ends

Reproduce crash where synthetic revs break merge detection (issue1578). (The fix for this was committed as 9bbcfa898cd3.) Slightly edited by Patrick Mezard <pmezard@gmail.com>

convert/svn: ignore composite tags Tools like cvs2svn generate tags made of files coming from different revisions from different branches. This is not supported by Mercurial, and it slows down the conversion a lot. Ignore them. See bacula@4082 for a sample.

Merge with crew-stable

convert/svn: keep latest and not oldest tag value

Don't use sed -r; instead use old-style regexp Rev d895158fe8af introduced some sed -r tests, but -r is only available on GNU sed, while BSD sed uses -E. Better to use old-style regular expressions, that way the tests work on all sed variants.