Write out hgrc properly.
Previously, we simply appended to the hgrc file, which meant that it
ended up containing multiple "paths" sections. Now, we only modify
"paths.default".
1:3aa14bbc23d90:8633637036c13aa14bbc23d9 tip8633637036c18633637036c1+8633637036c13aa14bbc23d9 tipmerging file13aa14bbc23d9 tip3aa14bbc23d9 tip3aa14bbc23d9 tip3aa14bbc23d9 tip