git-gui: Cleanup end-of-line whitespace in commit messages.

When committing changes its useless to have trailing whitespace on the
end of a line within the commit message itself; this serves no purpose
beyond wasting space in the repository.  But it happens a lot on my
Mac OS X system if I copy text out of a Terminal.app window and paste
it into git-gui.

We now clip any trailing whitespace from the commit buffer when loading
it from a file, when saving it out to our backup file, or when making
the actual commit object.

I also fixed a bug where we lost the commit message buffer if you quit
without editing the text region.  This can happen if you quit and restart
git-gui frequently in the middle of an editing session.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -428,6 +428,7 @@ proc load_message {file} {
 		}
 		set content [string trim [read $fd]]
 		close $fd
+		regsub -all -line {[ \r\t]+$} $content {} content
 		$ui_comm delete 0.0 end
 		$ui_comm insert end $content
 		return 1
@@ -1046,6 +1047,7 @@ You must add at least 1 file before you can commit.
 	# -- A message is required.
 	#
 	set msg [string trim [$ui_comm get 1.0 end]]
+	regsub -all -line {[ \t\r]+$} $msg {} msg
 	if {$msg eq {}} {
 		error_popup {Please supply a commit message.
 
@@ -2984,12 +2986,13 @@ proc do_quit {} {
 	#
 	set save [gitdir GITGUI_MSG]
 	set msg [string trim [$ui_comm get 0.0 end]]
-	if {![string match amend* $commit_type]
-		&& [$ui_comm edit modified]
+	regsub -all -line {[ \r\t]+$} $msg {} msg
+	if {(![string match amend* $commit_type]
+		|| [$ui_comm edit modified])
 		&& $msg ne {}} {
 		catch {
 			set fd [open $save w]
-			puts $fd [string trim [$ui_comm get 0.0 end]]
+			puts -nonewline $fd $msg
 			close $fd
 		}
 	} else {