scmutil: speed up relativization of paths when it's a no-op
Running commands from the root is commmon, in particular for
automation. Running `hg files > /tmp/a` from the root of
mozilla-central on linux:
before:
real 0m1,510s
user 0m1,387s
sys 0m0,090s
after:
real 0m1,266s
user 0m1,165s
sys 0m0,073s
(there are 280k paths, so this was costing ~1us per path somehow)
Differential Revision: https://phab.mercurial-scm.org/D8585
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true