Move UnprocessedCommitError to core
Turn thread pool size into a constant in history builder
Remove unused parameter in deduce_locale
Refactor translation history builder

The translation history builder walks over all commits looking for
changes to source files (i.e. files that contain english phrases). It
keeps track of source changes and continues searching through the
commit history looking for target changes (changes to files that
contain translations). When another source change is found, it extracts
translations from the current target files and associates them with the
current source phrases.