release: merge back stable branch into default

Added tag v4.15.0 for changeset 6c6f49fda019

release: Finish preparation for 4.15.0

release: Merge default into stable for release preparation

dependencies: bumped pygments to 2.3.0 release.

mercurial: use consistent name for backend

git: dropped some unused code to calculate path history

hooks: add extra debug/logging to help track hook errors.

code: fix deprecated octal usage for py3 compatability.

code: format style and py3 compatability