When we use the one panel for the entire commit message, its tricky to have a keybinding both for adding a newline and submitting.
By having two panels: one for the summary line and one for the description, we allow for 'enter' to submit the message when done from the summary panel,
and 'enter' to add a newline when done from the description panel. Alt-enter, for those who can use that key combo, also works for submitting the message
from the description panel. For those who can't use that key combo, and don't want to remap the keybinding, they can hit tab to go back to the summary panel
and then 'enter' to submit the message.
We have some awkwardness in that both contexts (i.e. panels) need to appear and disappear in tandem and we don't have a great way of handling that concept,
so we just push both contexts one after the other, and likewise remove both contexts when we escape.
When using the "copy commit message to clipboard" action, the message will end
up in the clipboard with duplicate line breaks. The same issue also affects the
"Reword Commit" command. GetCommitMessage(), the function used to retrieve the
commit message first splits the output returned by git into separate lines -
without removing the line breaks. After removing the first line (which contains
the commit SHA), it joins the lines of the message itself back together - adding
a second set of line breaks along the way. Stop this from happening.
Fixes#1808.
