* Document dvc.scm.git::Git::_get_diff_trees and shorten commit hashes in dvc.scm.git::Git::get_diff_trees
* diff: Changes to dvc.scm.git so `dvc diff` outputs short SHAs
- Docstring for Git class constructor
- Renames self.git for self.repo throughout module (involves several test changes)
- Better doc and logical refactor in Git._get_diff_trees
- `dvc diff` now outputs short SHA strings (Git._get_diff_trees) - hard coded to 7 length str
(involves some test changes in tests/func/test_diff.py)
For #1902
* Undoes "Renames self.git for self.repo throughout module" introduced in 3d2dc83.
* diff: `dvc diff` uses code for `git rev_parse --short` instead of hard `[:7]` (for short commit hashes).
- Tests updated too.
Per https://github.com/iterative/dvc/pull/1906#discussion_r278510340
* tests: Reformatted with `black tests/func/test_diff.py`
* tests: Remove uneccessary `str()` casting form `dvc diff` tests (and reformat)
Jorge Orpinel
Ruslan Kuprieiev