buildhistory: add script to check for significant changes
Adds a buildhistory-diff script which can be used to analyse changes in
the buildhistory git repository (as produced by buildhistory.bbclass),
and report significant ones that may need manual checking to ensure they
aren't regressions (e.g. package size changed by more than a certain
percentage, files added/removed/changed in the image, etc.)
The implementation is actually split into a small script and a Python
module, in order to make the logic re-usable in a future web-based
interface.
Implements the first part of [YOCTO #1566].
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
