help: explain what the term "revset" means
We refer to revsets in a few places (e.g. in "hg help config"), but we
never explained what they are. Until now.
--- a/mercurial/help/revisions.txt Wed Jan 11 11:37:38 2017 -0800
+++ b/mercurial/help/revisions.txt Wed Jan 11 22:46:07 2017 -0800
@@ -33,7 +33,7 @@
=============================
Mercurial supports a functional language for selecting a set of
-revisions.
+revisions. Expressions in this language are called revsets.
The language supports a number of predicates which are joined by infix
operators. Parenthesis can be used for grouping.