Tag keyword (keyword extension)

[Matt Mackall]

> What do you mean by "expensive"? 

Because commits are an atomic repo-wide operation in Mercurial,
everywhere you use a "tag" keyword ought to say 1.0 when you check out
1.0. Thus, when we check out 1.0, even if it only means updating a
couple files in our working directory (because we were at 0.999
already), we may have to scan and modify all the tens of thousands of
files in our project. A sub-second operation now takes minutes or more.

Fortunately, there are better ways to do all the sensible things you
might want to do with keywords, so there's really no reason to use them
in the first place.

-- 
Mathematics is the supreme nostalgia of our time.