Fix #38: properly interpret the saved --edit-msg argument

The `is` operator compares "pointers" a la C, the regular `==` compare
operator must be used to compare the contents of the strings.
rebase: Improve warning message when not mergeable
rebase: Add --pause option to pause before pushing

This option is particularly useful for testing, as it pauses the
`pull rebase` command just after the actual rebase was successful, before
the rebased code is pushed and the issue is closed.

Closes #39.