rebase -i: clarify and fix 'fixup -c' rebase-todo help

rebase -i: clarify and fix 'fixup -c' rebase-todo help

When `-c` says "edit the commit message" it's not clear what will be
edited. The original's commit message or the replacement's message or a
combination of the two. Word it such that it states more precisely what
exactly will be edited. While at it, also drop the jarring period and
capitalization, neither of which is otherwise present in the message.

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Charvi Mendiratta <charvi077@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>