rbt post has a lot of duplication of logic when it comes to building

diffs.  There are two functions responsible for diff building:

_get_diff_history() and _get_squashed_diff(). Both build the same

dictionary of common keyword arguments up-front. _get_diff_history()

then proceeds to make multiple diff() calls and passes in (and

re-computes) values for each call, rather than putting those into the

common dictionary.
This change introduces _build_get_diff_kwargs(), which computes all

actual common arguments. These functions then call them and pass the

results into each diff() call. This simplifies their code

considerably, and should help avoid issues in the future.

Tested posting squashed and history diffs for review locally. Posted

this change as well.
rbt post unit tests pass.