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>

rebase-interactive.c

@@ -44,9 +44,10 @@ void append_todo_help(int command_count,
 "r, reword <commit> = use commit, but edit the commit message\n"
 "e, edit <commit> = use commit, but stop for amending\n"
 "s, squash <commit> = use commit, but meld into previous commit\n"
-"f, fixup [-C | -c] <commit> = like \"squash\", but discard this\n"
-"                   commit's log message. Use -C to replace with this\n"
-"                   commit message or -c to edit the commit message\n"
+"f, fixup [-C | -c] <commit> = like \"squash\" but keep only the previous\n"
+"                   commit's log message, unless -C is used, in which case\n"
+"                   keep only this commit's message; -c is same as -C but\n"
+"                   opens the editor\n"
 "x, exec <command> = run command (the rest of the line) using shell\n"
 "b, break = stop here (continue rebase later with 'git rebase --continue')\n"
 "d, drop <commit> = remove commit\n"
@@ -55,7 +56,7 @@ void append_todo_help(int command_count,
 "m, merge [-C <commit> | -c <commit>] <label> [# <oneline>]\n"
 ".       create a merge commit using the original merge commit's\n"
 ".       message (or the oneline, if no original merge commit was\n"
-".       specified). Use -c <commit> to reword the commit message.\n"
+".       specified); use -c <commit> to reword the commit message\n"
 "\n"
 "These lines can be re-ordered; they are executed from top to bottom.\n");
 	unsigned edit_todo = !(shortrevisions && shortonto);