Ensure proper footer sorting of footer

When no trailer was configured in git configuration, TRAILERS_BY_REGEX
remained empty. This caused the subsequent match condition to always
evaluate to true, leading to incorrect footer sorting regardless of
their content. To address this issue, the update ensures that when the
configuration is empty, default terms are used to initialize
TRAILERS_BY_REGEX. The sorting mechanism is then adjusted to prioritize
other footers, configured trailers, and the change-id.

Co-authored-by: EricccTaiwan <yphbchou0911@gmail.com>
Change-Id: I6df5438b7ec6c85f5438f869b44cd14589c03339

Author: charliechiou

scripts/commit-msg.hook

@@ -151,6 +151,16 @@ build_commit_trailer_regex() {
     "Git-Dch[$separators] (Ignore|Short|Full)"
   )
 
+  default_trailers_by=(
+    'Signed-off-by'
+    'Reviewed-by'
+    'Co-authored-by'
+    'Acked-by'
+    'Suggested-by'
+    'Tested-by'
+    'Reported-by'
+  )
+
   # Read custom trailer keys from git config and add them either to specials or trailers.
   # This loop reads lines matching 'trailer.*.key'.
   while read -r _ key; do
@@ -170,6 +180,10 @@ build_commit_trailer_regex() {
     fi
   done < <(git config --get-regexp 'trailer.*.key')
 
+  if [[ ${#trailers_by[@]} -eq 0 ]]; then
+    trailers_by=("${default_trailers_by[@]}")
+  fi
+
   # Possible trailers :
   # - Acked-by
   # - Co-authored-by
@@ -178,8 +192,7 @@ build_commit_trailer_regex() {
   # - Signed-off-by
   # - Suggested-by
   # - Tested-by
-  TRAILERS_BY_REGEX="^($(IFS='|'; echo "${trailers_by[*]}")):"
-
+  TRAILERS_BY_REGEX="/($(IFS='|'; echo "${trailers_by[*]}" | tr '[:upper:]' '[:lower:]'))/"
   # Begin constructing the regex.
   TRAILER_REGEX='^('
 
@@ -610,7 +623,7 @@ add_change_id() {
     other_footer = ""
 
     for (line = 1; line <= numlines; line++) {
-      if (match(tolower(footer[line]), TRAILERS_BY_REGEX)) {
+      if (match(tolower(footer[line]), '"$TRAILERS_BY_REGEX"')) {
         trailers = trailers footer[line] "\n"
       } else {
         other_footer = other_footer footer[line] "\n"