Change 'pick' to 'reword' for single-commit "squashes"

When dealing with commits "combined" from a single commit, previously
we set the action to 'pick' in the sequence, with the following
consequences:
1. The commit messages of these commits aren't changed, even if they
   contain the special symbols we support.
2. The count of commits after the rebase is wrong.

Now we use 'reword' instead of 'pick' in this case, and therefore
solve these problems.
A minor bugfix
Use Popen() instead of check_output() to run git-rebase(1)

This way we can pipe git-rebase(1)'s output to stdout in real-time,
instead of printing them at once when it finishes.
Bump version to 0.1.0