Re: t5515 in general

In general I would agree that the whole "backward compatible"
thing we do with .git/branches and then .git/remotes when we
really went to the new model of using .git/config primarily was
done too cautiously, in that the configuration would not affect
existing setup that used branches and remotes, as they did not
have any way to express those new concepts (i.e. per branch
default remote, and per branch default merge sources).

I am actually in favor of killing .git/branches support way
before we do 1.6.0, and .git/remotes by the time we do 1.6.0.
But if you can arrange the url/fetch configurations to be still
taken from these old locations while applying per-branch default
remote and merge from .git/config, that would probably be easier
for people to migrate to .git/config only style.