[lint] improve handling of escaped % in lint-format-strings

Summary:
The linter considered 2 or more consecutive "%" as an escaped "%" character.
This fixes the problem: now the linter escapes only even numbers of "%" characters.

Test Plan:
Run unit tests (via doctest):

test/lint/lint-format-strings.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13319