Emacs: Indent first header line only when indentation is turned on

Previously in message-show mode message's first header line (From
header) was always indented, even if user had turned thread
indentation off with "<" (notmuch-show-toggle-thread-indentation)
command.

This change modifies notmuch-show-insert-headerline function so that
it doesn't indent the first header line if notmuch-show-indent-content
variable is nil.

This change also modifies tests so that they expect this new output
format:
test/emacs-show.expected-output/notmuch-show-indent-thread-content-off