Normalise LCOV source paths (#373)

Under windows, the git paths use unix-style paths, while the lcov paths use windows-style paths. In addition, coverage.py uses relative paths, but other tools like llvm-cov report absolute paths. So the source paths need to be made relative and unix-style.

diff_cover/violationsreporters/violations_reporter.py

@@ -297,7 +297,7 @@ def parse(lcov_file):
             # we're only interested in file name and line coverage
             if directive == "SF":
                 # SF:<absolute path to the source file>
-                source_file = content
+                source_file = util.to_unix_path(GitPathTool.relative_path(content))
                 continue
             elif directive == "DA":
                 # DA:<line number>,<execution count>[,<checksum>]