cache negative lookups of global numcopies and mincopies

Speeds up eg git-annex sync --content by up to 50%. When it does not need
to transfer or drop anything, it now noops a lot more quickly.

I didn't see anything else in sync --content noop loop that could really
be sped up. It has to cat git objects to keys, stat object files, etc.

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doc/bugs/importtree_spends_hours_reading_cidsdb/comment_14_e961d2b508e3260670bb61a5ae51d109._comment

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+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 14"""
+ date="2023-06-06T17:11:35Z"
+ content="""
+There's only one import in the sync, and your output shows it completed
+(with error).
+
+The only other phase of sync that could be run after that and take a lot of
+time is content syncing. You would have to have annex.synccontent set
+somewhere for sync to do that. Do you?
+"""]]

doc/bugs/importtree_spends_hours_reading_cidsdb/comment_15_35d233db5f7410048bd0cf9f5b4fd5cc._comment

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+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 15"""
+ date="2023-06-06T17:31:49Z"
+ content="""
+It would make a lot of sense for --content syncing to be what remains slow.
+That has to scan over all the files and when it decides that it does not
+need to copy the content anywhere, that's a tight loop with no output.
+
+In my repo with 10000 files that was set up by the latest test case,
+`git-annex sync` takes 13 seconds, and with --content it takes 61 seconds.
+
+I optimised a numcopies/mincopies lookup away, and that got it
+down to 28 seconds. 
+
+The cidsdb does not get accessed by the --content scan
+in my testing, although there may be other situations where it does.
+"""]]