merge: use collections.defaultdict() for mergeresult.commitinfo
We will be storing info from mergeresult.commitinfo to mergestate._stateextras
in upcoming patches, let's make them use same structure so that we don't have to
make much efforts in transferring info from one to other.
Differential Revision: https://phab.mercurial-scm.org/D8921
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true