extremely slow log parsing with large repository

[mgood]

Since the SourceFileLinkFormatter walks the whole repository branch used for the build, it becomes slower the larger the repository is. On a repository branch of about 4000 nodes it takes about 10 seconds to render.

[cmlenz]

This should get fixed for 0.5.

[cmlenz]

I've made some changes for this in [268]. Please test to see if that helps.

Now the log formatter will only match file references with line numbers, in the commonly used format filepath:lineno. It no longer walks the entire repository tree for every log... rather, for every file reference, it'll check to see whether there's a corresponding node in the repository. It also uses a path segment cache to avoid redundant lookups.