make savemessagelabels honor utime_from_header

Previously, syncing labels on a message always resulted in updating the
file modification time, even with utime_from_headers=true

This patch restores the file mtime to the previous value when
utime_from_headers=true, preventing a label synchronization from
breaking the promise that the file mtimes coincide with the header date.

Signed-off-by: Abdo Roig-Maranges <abdo.roig@gmail.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/GmailMaildir.py

@@ -159,7 +159,7 @@ class GmailMaildirFolder(MaildirFolder):
         content = self.deletemessageheaders(content, self.labelsheader)
         content = self.addmessageheader(content, '\n', self.labelsheader, labels_str)
 
-        rtime = self.messagelist[uid].get('rtime', None)
+        mtime = long(os.stat(filepath).st_mtime)
 
         # write file with new labels to a unique file in tmp
         messagename = self.new_message_filename(uid, set())
@@ -174,8 +174,9 @@ class GmailMaildirFolder(MaildirFolder):
               (tmppath, filepath, e[1]), OfflineImapError.ERROR.FOLDER), \
               None, exc_info()[2]
 
-        if rtime != None:
-            os.utime(filepath, (rtime, rtime))
+        # if utime_from_header=true, we don't want to change the mtime.
+        if self.utime_from_header and mtime:
+            os.utime(filepath, (mtime, mtime))
 
         # save the new mtime and labels
         self.messagelist[uid]['mtime'] = long(os.stat(filepath).st_mtime)