Add ![] to acceptable tag characters
Support test suite execution on Python 2.6 and earlier

This is to support RHEL/CentOS 6.x which only has Python 2.6.
subprocess.check_output() is new in Python 2.7.  Use subprocess.Pipe()
instead.

$ make t_regress
== Pathological cases ==
  t9601 ## Test handling of vendor branches
  t9602 ## Test handling of pathological tags
2,3c2,9
< t9602 test of branch vendorbranch: tag or branch vendorbranch unexpectedly missing
< t9602 test of tag vendortag: trees unexpectedly match

Signed-off-by: Eric S. Raymond <esr@thyrsus.com>
NEWS update.
Avoid a core dump when a commit cannot be assigned.