When pasting a multi-line commit message into the subject field of the commit
editor, we would interpret the first newline as the confirmation for closing the
editor, and then all remaining characters as whatever command they are bound to,
resulting in executing all sorts of arbitrary commands.
Now we recognize this being a paste, and interpret the first newline as moving
to the description.
Also, prevent tabs in the pasted content from switching to the respective other
panel; simply insert four spaces instead, which should be good enough for the
leading indentation in pasted code snippets, for example.
ba6cfc1f85