Use ~ instead of $HOME in .hgrc.

Summary: Use ~ instead of $HOME in .hgrc.

Test Plan:
1. Created a test commit.
2. Ran hg commit; looked for the boilerplate.
3. Edited the test plan, but left the default summary in to trigger the post-commit hook: "Must enter a summary in the commit message."
4. Ran hg commit again; wrote a proper commit message with summary and test plan.
5. Ran hg push and noticed it succeeded.

(The above steps were executed on a Mac only.)

Reviewers: alpert

Reviewed By: alpert

CC: david, benkomalo

Differential Revision: http://phabricator.khanacademy.org/D1004