Obomerge on consider and replaced by tags

SF1871096 - consider tags
SF1871092 - consider tags
SF1813702 - consider tags
also SF 2051721 - obomerge loses replaced_by tags

This is a summary of the bug:

parent:

format-version: 1.0
date: 06:10:2006 11:30 

[Term]
id: GO:0000001
name: live term to be obsoleted

[Term]
id: GO:0000002
name: This term will stay live 

live:  

format-version: 1.0
date: 06:10:2006 11:30

[Term]
id: GO:0000001
name: live term to be obsoleted (now obsolete)
is_obsolete: true
replaced_by: GO:0000002 

[Term]
id: GO:0000002
name: This term will stay live

branch: 

format-version: 1.0
date: 06:10:2006 11:30 

[Term]
id: GO:0000001
name: live term to be obsoleted 

[Term]
id: GO:0000002
name: This term will stay live

output: 

format-version: 1.2
date: 23:02:2009 11:35 

[Term]
id: GO:0000001
name: live term to be obsoleted (now obsolete)
is_obsolete: true 
                      <----replaced_by tag missing

[Term]
id: GO:0000002
name: This term will stay live