Use posix slashes when looking up files from PR changeset

This fixes the "Skipping comment for file" warning that was indexing with paths with backslashes on windows.

post/clang_tidy_review/clang_tidy_review/__init__.py

@@ -548,7 +548,7 @@ def format_diff_line(diagnostic, offset_lookup, source_line, line_offset, line_n
     return code_blocks, end_line
 
 
-def try_relative(path):
+def try_relative(path) -> pathlib.Path:
     """Try making `path` relative to current directory, otherwise make it an absolute path"""
     try:
         here = pathlib.Path.cwd()
@@ -686,7 +686,7 @@ def create_review_file(
             notes=diagnostic.get("Notes", []),
         )
 
-        rel_path = str(try_relative(get_diagnostic_file_path(diagnostic, build_dir)))
+        rel_path = try_relative(get_diagnostic_file_path(diagnostic, build_dir)).as_posix()
         # diff lines are 1-indexed
         source_line = 1 + find_line_number_from_offset(
             offset_lookup,