Branch heads should not include "heads" that are ancestors of other heads.
For example, given 1 (branch a) -> 2 (branch b) -> 3 (branch a)
I expect "hg heads a" to show only 3.
Discovered by running hg heads HEAD on the mutt repo, where older clients
committed default on top of HEAD.
% pattern error
grep: invalid match pattern: nothing to repeat
% simple
port:4:export
port:4:vaportight
port:4:import/export
% all
port:4:4:-:spam:import/export
port:3:4:+:eggs:import/export
port:2:1:-:spam:import
port:2:2:-:spam:export
port:2:1:+:spam:export
port:2:2:+:spam:vaportight
port:2:3:+:spam:import/export
port:1:2:+:eggs:export
port:0:1:+:spam:import
% other
port:4:import/export
% follow
port:0:import
port2:6:4:+:eggs:deport
port:4:4:-:spam:import/export
port:3:4:+:eggs:import/export
port:2:1:-:spam:import
port:2:2:-:spam:export
port:2:1:+:spam:export
port:2:2:+:spam:vaportight
port:2:3:+:spam:import/export
port:1:2:+:eggs:export
port:0:1:+:spam:import
color:3:orange
color:3:+:orange
color:2:-:orange
color:1:+:orange
% match in last line without newline
adding noeol
% last character omitted in output to avoid infinite loop
noeol:4:no infinite loo
% issue 685
adding color
colour:1:octarine
color:0:octarine
colour:1:octarine
% issue 337
adding color
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
created new head
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
color:3:-:red
color:1:+:red