hgtagsfnodes: refactor code to compute fnode into separate fn
I plan to use this code at one more place while fixing a bug caused by an
invalid fnode present in cache.
Differential Revision: https://phab.mercurial-scm.org/D10013
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true