Manifest: Make sure manifest timestamp is strictly monotonically increasing.

Computer clocks are often not set very accurately set, but borg
assumes manifest timestamps are never going back in time.

Ensure that this is actually the case.

# Conflicts:
#	src/borg/helpers.py

Original-Commit: 6b8cf0a

src/borg/helpers.py

@@ -200,6 +200,7 @@ class Manifest:
         self.repository = repository
         self.item_keys = frozenset(item_keys) if item_keys is not None else ITEM_KEYS
         self.tam_verified = False
+        self.timestamp = None
 
     @property
     def id_str(self):
@@ -245,7 +246,13 @@ class Manifest:
         from .item import ManifestItem
         if self.key.tam_required:
             self.config[b'tam_required'] = True
-        self.timestamp = datetime.utcnow().isoformat()
+        # self.timestamp needs to be strictly monotonically increasing. Clocks often are not set correctly
+        if self.timestamp is None:
+            self.timestamp = datetime.utcnow().isoformat()
+        else:
+            prev_ts = datetime.strptime(self.timestamp, "%Y-%m-%dT%H:%M:%S.%f")
+            incremented = (prev_ts + timedelta(microseconds=1)).isoformat()
+            self.timestamp = max(incremented, datetime.utcnow().isoformat())
         manifest = ManifestItem(
             version=1,
             archives=StableDict(self.archives.get_raw_dict()),