Below are the steps that I used to move my repos:
1. Create an empty git repo in both github as well as bitbucket for the merucrial repo in bitbucket that you want to migrate.
2. Enable hg-git tool and commit to the newly created git repo in bitbucket.
Say for example, your mericurial repo is foo and and git repo is foo-git, the command would look like this:
C:\Personal\Projects\foo>hg push git+ssh://email@example.com:abhirama/foo-git.git
3. Now create a folder called foo and clone the bitbucket git repo to this folder.
C:\Personal\Projects\git>git clone https://firstname.lastname@example.org/abhirama/foo-git.git .\foo
4. Merge changes from github repo foo to the repo foo cloned from bitbucket.
C:\Personal\Projects\git\foo [master]> git pull email@example.com:abhirama/foo.git
5. Once, done, change the origin of this repo from bitbucket to github.
C:\Personal\Projects\git\foo [master]> git remote rm origin
C:\Personal\Projects\git\foo [master]> git remote add origin firstname.lastname@example.org:abhirama/foo.git
6. Now push this repo to github.
C:\Personal\Projects\git\foo [master]> git push –set-upstream origin master
Voila, your repo is in github now :).