[PATCH 0/8] Improve tag change completion

[Austin Clements]

This series improves tag change completion in various ways for
commands like +, -, and *.

>From a user perspective, this provides command-specific prompts like
"Tag message" and "Tag all" instead of the generic "Tag" prompt, and
bases tag removal completions on the tags that are in the buffer,
rather than the current tags in the database, providing a more
predicable experience.

>From an implementation perspective, this new tag removal completion
behavior improves efficiency and eliminates a road block to fixing the
tagging race bug (which otherwise results in massive queries just to
compute removal completions).  The new code is also more "Elispy" and
predictable because all tag change prompting now occurs at the
interactive entry points, rather than buried under several layers of
non-interactive calls.

This is a spiritual successor to
id:1354263691-19715-1-git-send-email-markwalters1009 at gmail.com, though
it takes a very different approach.  This is also a prerequisite to
the tag race fix in
id:1381185201-25197-1-git-send-email-amdragon at mit.edu and I plan to
send an updated version of that series when this one is accepted.

Patches 1, 5, and 6 could be pushed on their own.  They fix bugs or
sort of bugs that get in the way of the rest of the series.