merge: when current branch has 1 or > 2 heads, actually abort.
Currently merge just prints abort-like messages to stderr and then
exits with a misleading status 0 (cleverly disguised as "False").
With this change it raises Abort, just like every other fatal error.
% hg debuginstall
Checking encoding (ascii)...
Checking extensions...
Checking templates...
Checking patch...
Checking commit editor...
Checking username...
No problems detected
% hg debuginstall with no username
Checking encoding (ascii)...
Checking extensions...
Checking templates...
Checking patch...
Checking commit editor...
Checking username...
no username supplied (see "hg help config")
(specify a username in your .hgrc file)
1 problems detected, please check your install!