gitk: visually indicate whether a tag exists on a remote

[Adi Shavit <adishavit@xxxxxxxxx>]

Hi all,

*Problem*

When browsing history in **gitk**, I often need to know whether a tag I
see on a commit was:

  * fetched from a remote (and therefore shared with collaborators), or
  * created only in my local repository and not yet pushed.

For branches this distinction is obvious because gitk lists local and
remote-tracking branch heads separately.  Tags, however, are shown in a
single list, so a locally added tag and one fetched from ­–‐say–‐
`origin` look identical.  This can mislead users into thinking a tag is
already published when in fact it is not.

*Proposed enhancement*

Add an opt-in visual cue that a tag also exists on at least one remote.
Possible UI sketches (open for discussion):

  1. **Suffix** the tag label with the remote name in parentheses, e.g.
     `v1.2.3 (origin)`.
  2. **Colour** or style the tag differently (similar to how remote
     branches are blue vs. local green).
  3. **Tool-tip**: on hover, list the remotes that advertise the tag.

*Why this belongs in gitk*

gitk’s strength is that it gives an at-a-glance view of history.  Making
tags’ publication state equally transparent would save users a trip to
the command line and reduce accidental “forgot to push the tag” issues.

Thanks for considering, and for maintaining gitk!

Best regards,
Adi Shavit