Use strbufs in commit.c (pretty printing)

  Here is a 3 patch series to use strbufs for the pretty printing
functions in commit.c.

  * The first one adds a needed useful function to strbufs (rtrim).
  * The second one changes the semantics of interpolate (as the third
    will need that) and fixes the rest of the code accordingly.
  * Do the change.

  Testsuite passes after step 2 and 3.