Fix detection of non-executable lines for reporting to Phabricator and notifying low coverage patches

Before f0a6558, we were parsing reports in the codecov/coveralls format, where
None meant a non-executable lines. In the covdir format instead, a non-executable line is marked with -1.

The regression went unnoticed because our tests were using the codecov/coveralls format and, instead of rewriting
them to use the covdir format directly (which is harder to read for a human), we introduced a test utility function
to convert them automatically, which had a bug (we did not convert None to -1).

Fixes a regression from f0a6558

Author: marco-c

bot/code_coverage_bot/phabricator.py

@@ -108,7 +108,7 @@ def _apply_coverage_map(self, annotate, coverage_map):
             key = "{}-{}".format(orig_changeset, orig_line)
             if key in coverage_map:
                 count = coverage_map[key]
-                if count is None:
+                if count == -1:
                     # A non-executable line.
                     phab_coverage_data += "N"
                 elif count > 0: