gitconfig: git config --global format.signOff true

Summary:
Ref [man git config](https://git-scm.com/docs/git-config#Documentation/git-config.txt-formatsignOff).

Also disables aliases with `-s` now that config takes over

Signed-off-by: Yongmin Hong <revi@omglol.email>

Test Plan: Apply patch and `git cm`

Reviewers: O1 revi & automations, revi

Reviewed By: O1 revi & automations, revi

Differential Revision: https://issuetracker.revi.xyz/D100

.gitaliases

@@ -2,7 +2,7 @@
 # SPDX-License-Identifier: Apache-2.0
 [alias]
   co = checkout
-  cm = commit -s
+  cm = commit
   st = status
   br = branch -v
   fa = fetch --all
@@ -21,11 +21,10 @@
   por = push origin master
   pon = push origin main
   amend = commit --amend
-  amends = commit --amend -s
-  cmt = commit -S -s
-  amendss = commit --amend -S -s
-  acmt = commit -a -S -s
-  acm = commit -a -s
+  cmt = commit -S
+  amends = commit --amend -S
+  acmt = commit -a -S
+  acm = commit -a
   fup = fetch upstream
   com = !echo 'ERROR: use cor for master, and con for main.'
   comr = checkout master

default.gitconfig

@@ -22,6 +22,8 @@
   smudge = git-lfs smudge -- %f
   process = git-lfs filter-process
   required = true
+[format]
+  signOff = true
 [sendemail]
   from = 'Yongmin Hong <revi@omglol.email>'
   smtpserver = smtp.fastmail.com