This change reworks the code that handles creating a commit when

patching (using rbt patch -c) or landing (using rbt land -e) to

improve the editing experience and error handling.
Both Git and Mercurial provide a custom filename for the editor,

mirroring the format of the filenames normally used by those tools. This

gives editors a change to perform syntax highlighting, line length

limits, etc.
If the user deletes the entire contents of the commit message, the

commit won't be created. Instead, a message will be shown explaining the

situation and next steps.
There are also some fixes for command line arguments for specifying

author information for Git and Mercurial, improved error handling all

throughout, and better file tracking in Mercurial.
In addition, we now have extensive test coverage for commit creation for

both Git and Mercurial.

Unit tests pass for Git and Mercurial.
Manually tested locally patching changes from both Git and Mercurial

in the commit editor mode. Verified that the correct message was used

when editing the commit, and that a suitable set of instructions was

provided when canceling the commit.
Also verified tracking of edited and new files in Mercurial.