revlog: give formatting to clang-format
This has two lines that are poorly formatted, both right after
PyObject_HEAD macros. It's possible that in the future there will be a
feature to tell clang-format that a token includes an end-of-line
character (in this case the ;), but for now I'm willing to live with
two poorly-formatted lines in a struct definition in exchange for not
thinking about how this file is formatted.
Differential Revision: https://phab.mercurial-scm.org/D5234