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.
% wide diffstat
a | 213 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 files changed, 213 insertions(+), 0 deletions(-)
% diffstat width
a | 213 ++++++++++++++
1 files changed, 213 insertions(+), 0 deletions(-)
% narrow diffstat
a | 3 +++
1 files changed, 3 insertions(+), 0 deletions(-)
% binary diffstat
b | 0
1 files changed, 0 insertions(+), 0 deletions(-)
% binary git diffstat
b | Bin
1 files changed, 0 insertions(+), 0 deletions(-)