obsolete: properly drop 'date' from metadata

Mistakes were made while resolving rebase conflicts in 4bc96685a40c. This led to
'date' being preserved in metadata when reading markers from a binary stream.

As a result, some known markers were seen as "new" when pulling. I noticed it
because a no-op pulls from main added about 600 duplicated markers to my
obsstore (for each pull).

I do not believe we need to perform any specific action to actively
de-duplicates existing obsstore. After this fix, duplicated markers will no be
propagated and the few affected repositories can probably deal with duplication (or
people can repull the obsstore from a clone).

As a side effect, we decode metadata only once, reducing the impact of the hack
in fm0 to store extra important data (parents and date).

mercurial/obsolete.py

@@ -181,7 +181,7 b' def _fm0readmarkers(data, off=0):'
         off += mdsize
         meta = decodemeta(metadata)
         try:
-            when, offset = decodemeta(metadata).pop('date', '0 0').split(' ')
+            when, offset = meta.pop('date', '0 0').split(' ')
             date = float(when), int(offset)
         except ValueError:
             date = (0., 0)