[MERGE] Merge types are provided by a registry

[John Arbash Meinel]

John Arbash Meinel has voted +1.
Status is now: Approved
Comment:
Looks good.

I know we disagree on this, but I honestly do prefer 
'--merge-type=diff3' to '--diff3'.

As someone else mentioned, doing '--merge-type=unknown' would give an 
unknown format, rather than an unknown argument.

I do realize that --diff3 is shorter. But I think it is proliferating 
options, and has the potential to cause conflicts (I realize it is 
unlikely, but it is sort of like using namespaces to keep things 
distinct).

Not that I expect us to ever agree :)

And I think this patch is better than our existing code, so +1.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C45C9FC5E.70207%40utoronto.ca%3E