patch: properly escape \ in string literals
Python 3.8 will emit a SyntaxWarning for str/bytes with invalid
escapes. This commit addresses 4 occurrences where we had a bare
\ in a str/bytes.
Differential Revision: https://phab.mercurial-scm.org/D5818
max_width = 79
wrap_comments = true
error_on_line_overflow = true