let the logger do the formatting

[sascha_silbe]

With the attached patch we delegate the string formatting to the logger instead of doing it ourselves.
Advantages:
a) It's more robust: Constructs like 'debug("foo %r bar" % x)' are quite common in our code and will break if x is a tuple. 'debug("foo %r bar", x)' will work regardless of the type of x.
b) formatting only happens if we actually log something.

I haven't measured the impact of b) because a) is much more important to me (happened a lot of times during the object_id => (tree_id, version_id) conversion). At least one project has asked for pylint to flag these occurences and gave performance as a reason for the change, though (so they mave have measured it).

[sascha_silbe]

let the logger do the formatting

[tomeu]

Looks good, we should push these changes early in the release cycle, though. Please push, thanks!

[sascha_silbe]

Pushed, thanks!