git-gui: strip the commit message after running commit-msg hook

When commit-msg writes the file using CRLF, the lines in the final
message include trailing spaces.

Postpone stripping until after hooks execute.

This aligns with Git's behavior, which passes the original message
to commit-msg, then strips comments and whitespace.

Signed-off-by: Orgad Shaneh <orgads@gmail.com>

lib/commit.tcl

@@ -208,28 +208,6 @@ You must stage at least 1 file before you can commit.
 	# -- A message is required.
 	#
 	set msg [$ui_comm get 1.0 end]
-	# Strip trailing whitespace
-	regsub -all -line {[ \t\r]+$} $msg {} msg
-	# Strip comment lines
-	global comment_string
-	set cmt_rx [strcat {(^|\n)} [regsub -all {\W} $comment_string {\\&}] {[^\n]*}]
-	regsub -all $cmt_rx $msg {\1} msg
-	# Strip leading and trailing empty lines (puts adds one \n)
-	set msg [string trim $msg \n]
-	# Compress consecutive empty lines
-	regsub -all {\n{3,}} $msg "\n\n" msg
-	if {$msg eq {}} {
-		error_popup [mc "Please supply a commit message.
-
-A good commit message has the following format:
-
-- First line: Describe in one sentence what you did.
-- Second line: Blank
-- Remaining lines: Describe why this change is good.
-"]
-		unlock_index
-		return
-	}
 
 	# -- Build the message file.
 	#
@@ -332,7 +310,52 @@ proc commit_commitmsg_wait {fd_ph curHEAD msg_p} {
 	fconfigure $fd_ph -blocking 0
 }
 
+proc wash_commit_message {msg} {
+	# Strip trailing whitespace
+	regsub -all -line {[ \t\r]+$} $msg {} msg
+	# Strip comment lines
+	global comment_string
+	set cmt_rx [strcat {(^|\n)} [regsub -all {\W} $comment_string {\\&}] {[^\n]*}]
+	regsub -all $cmt_rx $msg {\1} msg
+	# Strip leading and trailing empty lines (puts adds one \n)
+	set msg [string trim $msg \n]
+	# Compress consecutive empty lines
+	regsub -all {\n{3,}} $msg \n\n msg
+
+	return $msg
+}
+
 proc commit_writetree {curHEAD msg_p} {
+	# -- Process the commit message after hooks have run.
+	#
+	set msg_fd [safe_open_file $msg_p r]
+	setup_commit_encoding $msg_fd 1
+	set msg [read $msg_fd]
+	close $msg_fd
+
+	# Process the message (strip whitespace, comments, etc.)
+	set msg [wash_commit_message $msg]
+
+	if {$msg eq {}} {
+		error_popup [mc "Please supply a commit message.
+
+A good commit message has the following format:
+
+- First line: Describe in one sentence what you did.
+- Second line: Blank
+- Remaining lines: Describe why this change is good.
+"]
+		unlock_index
+		return
+	}
+
+	# Write the processed message back to the file
+	set msg_wt [safe_open_file $msg_p w]
+	fconfigure $msg_wt -translation lf
+	setup_commit_encoding $msg_wt
+	puts $msg_wt $msg
+	close $msg_wt
+
 	ui_status [mc "Committing changes..."]
 	set fd_wt [git_read [list write-tree]]
 	fileevent $fd_wt readable \