Fix aggregation in Windows (fix #210)

[devdanzin]

In Windows, aggregation is broken in two ways:

• Aggregated metrics on directories come only from files that changed in a given revision.
• Only root directories get aggregated entries (so src/ and test/ in wily's repo, but not src/wily/).

This happens because revision.tracked_files and revision.tracked_dirs store paths with forward slashes, but the paths in result use backslashes in Windows. This PR fixes that by converting entries in those lists to Paths and back to str.

This fixes the root cause of #210, which initially had a wrong analysis but documents the issue.

[codecov-commenter]

Codecov Report

Merging #211 (4149e8b) into master (b342a1c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #211   +/-   ##
=======================================
  Coverage   95.83%   95.84%           
=======================================
  Files          25       25           
  Lines        1346     1349    +3     
  Branches      287      289    +2     
=======================================
+ Hits         1290     1293    +3     
  Misses         32       32           
  Partials       24       24           

Files Changed	Coverage Δ
src/wily/commands/build.py	92.78% <100.00%> (+1.29%)	⬆️

... and 1 file with indirect coverage changes

[devdanzin]

Sorry about the noise, I couldn't get codecov to pass without adding tests, and I couldn't make the tests fail locally the same way they did on CI.