Added option to store hg hashes in git notes

On conversion original mercurial hashes would be added
to refs/notes/hg commit notes for each git commit if
--notes option was provided in the runtime.

Usage: pass --notes option to hg-fast-export.sh script.

Then on migrated git repo run git log command with --notes=hg option, e.g.:
```
$ git log --notes=hg 
commit 59913dd888c839763d176f1e34eb0b120a3fcffe
Author: Piotr Święcicki <swietopel.com>
Date:   Tue Apr 22 21:29:20 2014 +0200

    some content added

Notes (hg):
    46608a8e04b632e8b853e0553e00da39dace5bf4

commit 39002be4a33d4b3fecc40f1646107aa901941161
Author: Piotr Święcicki <swietopel.com>
Date:   Tue Apr 22 21:28:50 2014 +0200

    initial version

Notes (hg):
    d9718f700e42a0be54f285f5c083c145a337538f



$ hg log  
changeset:   1:46608a8e04b6
tag:         tip
user:        Piotr Święcicki <swietopel.com>
date:        Tue Apr 22 21:29:20 2014 +0200
summary:     some content added

changeset:   0:d9718f700e42
user:        Piotr Święcicki <swietopel.com>
date:        Tue Apr 22 21:28:50 2014 +0200
summary:     initial version
```