feat(git-commit-mode): add+update git-trailer snippets

- Add Co-authored-by: snippet
- Adds and updates old git trailer snippets to match Doom's git
  conventions (which are still recognized by Git{hub,lab}).

Ref: https://discourse.doomemacs.org/t/git-conventions/2407

git-commit-mode/amend

@@ -0,0 +1,5 @@
+# -*- mode: snippet -*-
+# name: Amend: ...
+# key: am
+# --
+Amend: ${1:URL|12-CHAR HASH|#GITHUBREF}

git-commit-mode/close

@@ -0,0 +1,5 @@
+# -*- mode: snippet -*-
+# name: Close: ...
+# key: cl
+# --
+Close: ${1:URL|12-CHAR HASH|#GITHUBREF}

git-commit-mode/co-authored-by

@@ -0,0 +1,5 @@
+# -*- mode: snippet -*-
+# name: Co-authored-by: ...
+# key: cab
+# --
+Co-authored-by: ${1:username} <${2:$1@users.noreply.github.com}>

git-commit-mode/fixes

@@ -1,6 +1,5 @@
 # -*- mode: snippet -*-
-# name: fixes
+# name: Fix: ...
 # key: fix
-# uuid: fix
 # --
-fixes #${1:100}
+Ref: ${1:URL|12-CHAR HASH|#GITHUBREF}

git-commit-mode/references

@@ -1,6 +1,5 @@
 # -*- mode: snippet -*-
-# name: references
+# name: Ref: ...
 # key: ref
-# uuid: ref
 # --
-references #${1:100}
+Ref: ${1:URL|12-CHAR HASH|#GITHUBREF}