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Improvement from Roman Shevchenko to be compatible with Ubuntu 13.10.
[log] Memory consumption: Hash -> int. Take 1

Store Hashes in a PersistentEnumerator.
Store only their int indices in memory.

Encapsulate TimedVcsCommit to the CompactCommit (which operates with
ints) right after receiving commits from the VCS.
Rebuild Hash objects when CompactCommit is accessed.

Known issues:
1. Since the Graph operates on Hashes, viewing full graph would
   unpack all Hash objects and consume a lot of memory.
2. Exceptions from the PersistentEnumerator should be handled more
   gracefully with ability to rebuild the map when corrupted.
[log] Don't call new VcsRefImpl() directly, use the factory
[log] Hash -> int in Graph

* Substitute Hashes by ints, VcsCommits with GraphCommits.
* Let CompactCommit implement graph's Commit to avoid objects
  creation and conversion.
* Supply hash-by-index and index-by-hash getters where needed.
* Temporary store index in VcsRef (will be removed later).
* Adjust tests by converting Hashes to int just by using
  Integer.parseInt(hash, 16) trimming the hash string to 4 digits (to
  fit into MAX_INT).
[log] Move SimpleHash from vcs-log-graph to vcs-log-impl

It is used only there.
temporary fix for layouts of small IDEs