Merge branch 'jc/combine-diff-many-parents'

We used to have an arbitrary 32 limit for combined diff input,
resulting in incorrect number of leading colons shown when showing
the "--raw --cc" output.

* jc/combine-diff-many-parents:
  t4038: add tests for "diff --cc --raw <trees>"
  combine-diff: lift 32-way limit of combined diff