histedit: add missing b prefix to a string
If i18n is disabled (such as via HGPLAIN=1), `_()` doesn't convert from str to
bytes, so this raises a TypeError on py3.
Differential Revision: https://phab.mercurial-scm.org/D8354
# 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