dgit-maint-{merge,debrebase}(7): Use git fetch --all --tags

dgit-maint-{merge,debrebase}(7): Use git fetch --all --tags

`git remote update` doesn't fetch tags unless remote branches include
the commits at which those tags point.  Thus, if upstream pushes their
release tag but fails to push their master branch, `git remote update`
will not fetch the release tag.

I've been in this situation more than once when following the
workflows detailed in these manpages, so let's just recommend a
command which will definitely try to fetch the latest release tag.

Closes: #939504
Signed-off-by: Sean Whitton <spwhitton@spwhitton.name>
Acked-by: Ian Jackson <ijackson@chiark.greenend.org.uk>