hgweb: make followlines button absolutely positioned
It used to have position: absolute only on annotate page, but it makes sense to
have it everywhere, because the button shouldn't affect other elements at all.
Especially since the button has a set height, which meant that for certain
smaller fonts source lines were changing their height on hover.
Note that the button doesn't set any of the usual properties that accompany
absolute position (top, right, bottom or left). These properties would position
the button without any account for source line padding. Instead, margins are
used (the button already has all margins defined, they do the job).
% lazy ancestor set for [], stoprev = 0, inclusive = False
membership: []
iteration: []
% lazy ancestor set for [11, 13], stoprev = 0, inclusive = False
membership: [7, 8, 3, 4, 1, 0]
iteration: [3, 7, 8, 1, 4, 0, 2]
% lazy ancestor set for [1, 3], stoprev = 0, inclusive = False
membership: [1, 0]
iteration: [0, 1]
% lazy ancestor set for [11, 13], stoprev = 0, inclusive = True
membership: [11, 13, 7, 8, 3, 4, 1, 0]
iteration: [11, 13, 3, 7, 8, 1, 4, 0, 2]
% lazy ancestor set for [11, 13], stoprev = 6, inclusive = False
membership: [7, 8]
iteration: [7, 8]
% lazy ancestor set for [11, 13], stoprev = 6, inclusive = True
membership: [11, 13, 7, 8]
iteration: [11, 13, 7, 8]