Added option to store hg hashes in git notes

After migrating hg to git repository it would be worth to have the mapping of mercurial commit hashes into git commit hashes handy. Git commit notes could be used for this.

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
```