git-deborig: new --just-print-tag-names option

The purpose of this is to give other programs access to git-deborig's
@candidate_tags list for a given upstream version.  This permits code
reuse, as we can maintain in one place the list of conventional ways
to choose the tag name for a new upstream version.

Closes: #931180
4 jobs for git-deborig-just-print-tag-names in 6 minutes and 58 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #217586
stable-bpo

00:06:45

passed #217585
testing

00:06:26

passed #217587
ubuntu-devel

00:06:25

passed #217584
unstable

00:06:58