Revisit commit message processing

[smashwilson]

Revisit the ways that we preprocess commit messages - both in JavaScript and in choosing a --cleanup argument to git commit - to better handle some edge cases and bugs.

• Fixes Ammend commit message is corrupted in certain cases since atom 1.27.0 #1465. It turns out that GitShellOutStrategy.getHeadCommit() was stripping newlines from the message body of any parsed commit, which was altering amended commit messages. Added --cleanup=verbatim as well to ensure that the message isn't manipulated by git either.
• Fixes Auto-wrap removed text from commit message #1382 by adding a {verbatim} option to GitShellOutStrategy.commit(). When set to true, the message argument is used for the commit exactly as-is. When set to false, if commit.cleanup is set to a value and is not "default", default to "strip".
  • Don't generate the "merge text" commit message template when a merge is detected (because we would no longer strip out the # lines).
  • Set {verbatim: true} when committing from the mini editor, but not from a full editor.

Note that I'm defaulting --cleanup to "strip" for non-verbatim commits instead of "whitespace". My logic here is that committing through Atom with a full-editor message feels more like editing a message in $EDITOR and committing than it does doing a commit -m, so we want to act like git "when the message is to be edited":

Same as strip if the message is to be edited. Otherwise whitespace.