filemerge: rename backup variables from `back` to `backup`
`backup` seems like an obviously clearer name for something containing
something related to a backup.
Differential Revision: https://phab.mercurial-scm.org/D11876
[build-system]
requires = ["setuptools", "wheel"]
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true