Add gti tag command to show available branches as licence plates

[BenBE]

Hey, I just wanted to say everything works fantastically.

Not sure why I even opened this issue.

I'm using gti on

• [ ] HP-UX running on a 68k
• [ ] NetBSD, with output piped to a slice of toast
• [ ] an unreleased beta version of Windows NT 4.0 with a cmd.exe replacement from ReactOS
• [X] something entirely different based on Linux

and it

• [ ] set fire to my line printer
• [ ] hard-linked /bin/emacs to/bin/vi
• [ ] crashes when run backwards in my home-grown CPU emulator
• [X] reliably produces output on my terminal.

Further improvements were possible for the gti tag command if it showed the list of available tags as a list of licence plates. The -v flag might add additional decorations, while the -l flag might show tags in large fidget font.

[rwos]

The basis for this is released in 1.7.0. Displaying the actual tag would just need a bit of arg parsing and printf magic sprinkled on top.