I have some old mercurial repositories, and recently I want to convert them into git repositories. It turns out that converting them is pretty easy. First, you need to get hg-fast-export:

git clone git://repo.or.cz/fast-export.git

Next, create a new git repository:

mkdir new_git_repo
cd new_git_repo
git init

Finally, use the hg-fast-export.sh script from the directory you just checked out in the first step:

/path/to/hg-fast-export.sh -r /path/to/hg_repo
git repack -a -d -f
git checkout BRANCH_NAME

BRANCH_NAME is the name of mercurial branch, the "default" branch in mercurial will be called "master".

hg-fast-export.sh supports --force parameter, you can use it if you get validation errors.