contrib/clang-format-ignorelist

rebase: extract a variable for a repeated `repo[p1]`

I'll add another use site in the next patch.

Differential Revision: https://phab.mercurial-scm.org/D7896

# Files that just need to be migrated to the formatter.
# Do not add new files here!
mercurial/cext/manifest.c
mercurial/cext/osutil.c
# Vendored code that we should never format:
syntax: glob
contrib/python-zstandard/**.c
contrib/python-zstandard/**.h
hgext/fsmonitor/pywatchman/**.c
mercurial/thirdparty/**.c
mercurial/thirdparty/**.h