help: move help topics from mercurial/help.py to help/*.txt
The help topics are loaded on demand so we wont hit the disk unless we
really have to.
When Mercurial accepts more than one revision, they may be
specified individually, or provided as a topologically continuous
range, separated by the ":" character.
The syntax of range notation is [BEGIN]:[END], where BEGIN and END
are revision identifiers. Both BEGIN and END are optional. If
BEGIN is not specified, it defaults to revision number 0. If END
is not specified, it defaults to the tip. The range ":" thus means
"all revisions".
If BEGIN is greater than END, revisions are treated in reverse
order.
A range acts as a closed interval. This means that a range of 3:5
gives 3, 4 and 5. Similarly, a range of 9:6 gives 9, 8, 7, and 6.