don't delete messages in local cache in dry-run mode

This makes dry-run consitent when called more than once.

Github-ref: https://github.com/OfflineIMAP/offlineimap/issues/370
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/Base.py

@@ -881,8 +881,7 @@ class BaseFolder(object):
                     self.ui.ignorecopyingmessage(uid, self, dstfolder)
 
         if num_to_copy > 0 and self.repository.account.dryrun:
-            self.ui.info(
-                "[DRYRUN] Copy {0} messages from {1}[{2}] to {3}".format(
+            self.ui.info("[DRYRUN] Copy {} messages from {}[{}] to {}".format(
                 num_to_copy, self, self.repository, dstfolder.repository)
             )
             return
@@ -947,13 +946,13 @@ class BaseFolder(object):
             # won't lose message, we will just unneccessarily retransmit some.
             # Delete messages from statusfolder that were either deleted by the
             # user, or not being tracked (e.g. because of maxage).
+            if not self.repository.account.dryrun:
                 statusfolder.deletemessages(deletelist)
-            # Filter out untracked messages
+            # Filter out untracked messages.
             deletelist = [uid for uid in deletelist if dstfolder.uidexists(uid)]
             if len(deletelist):
                 self.ui.deletingmessages(deletelist, [dstfolder])
-                if self.repository.account.dryrun:
-                    return #don't delete messages in dry-run mode
+                if not self.repository.account.dryrun:
                     dstfolder.deletemessages(deletelist)
 
     def combine_flags_and_keywords(self, uid, dstfolder):

offlineimap/folder/LocalStatusSQLite.py

@@ -24,6 +24,7 @@ import six
 
 from .Base import BaseFolder
 
+
 class DatabaseFileLock(object):
     """Lock at database file level."""
 
@@ -125,7 +126,7 @@ class LocalStatusSQLiteFolder(BaseFolder):
                     self.__upgrade_db(version)
 
     def purge(self):
-        """Remove any pre-existing database."""
+        """Remove any pre-existing database. Do not call in dry-run mode."""
 
         try:
             os.unlink(self.filename)
@@ -328,7 +329,7 @@ class LocalStatusSQLiteFolder(BaseFolder):
             # We cannot assign a uid.
             return uid
 
-        if self.uidexists(uid):     # already have it
+        if self.uidexists(uid):     # Already have it.
             self.savemessageflags(uid, flags)
             return uid