Fix a subtle issue with format-like strings in nibs

Nibs don't have comments (that appear in extracted strings), so we can't
force a string to be parsed as not-format-string. Previously it always
tried to parse as a format string first, which led to false positives
and mismatches from the corpus.

Fix this by checking if the corpus has an entry for the non-format
version; if so, parse as not-format-string; if not, use previous
behavior (parsing as format string if possible).