code: Make Code Browser much faster.

This reduce the time taken to show a root directory which has:

* 6 files and 4 subdirs in a git repository with 1923 commits,
  from 198ms to 55ms. (3.6x faster)
* 124 files in a git repository with 825 commits,
  from 6291ms to 135ms. (46x faster)
Merge pull request #264 from npcode/fix/code-browser-performance

code: Make Code Browser much faster.