make directory creation for history cache more efficient

[vladak]

As mentioned in PR #3636 the directory cache creation in FileHistoryCache#createDirectoriesForFiles() can be made more efficient. The first idea is to avoid mkdirs() with duplicate arguments. This might have impact especially for the initial indexing.

Another idea is to execute the mkdirs() calls in parallel. The synchronization done at file system level might make the impact of this negligible, however I don't like the idea of hammering the system with many syscalls at once.

These 2 approaches could be combined.

[vladak]

note to self: use opengrok/indexer/util/Progress.java to track the progress.