Preserve metadata when staging a new version of an annexed file.

Performance impact: When adding a large tree of new files, this needs
to do some git cat-file queries to check if any of the files already
existed and might need a metadata copy. I tried a benchmark in a copy
of my sound repository (so there was already a significant git tree
to check against.

Adding 10000 small files, with a cold cache:
  before: 1m48.539s
  after:  1m52.791s

So, impact is 0.0004 seconds per file added. Which seems acceptable, so did
not add some kind of configuration to enable/disable this.

This commit was sponsored by Lisa Feilen.

Annex/CatFile.hs

@@ -87,8 +87,7 @@ catKey' modeguaranteed ref mode
 		| modeguaranteed = catObject ref
 		| otherwise = L.take 8192 <$> catObject ref
 
-{- Looks up the file mode corresponding to the Ref using the running
- - cat-file.
+{- Looks up the key corresponding to the Ref using the running cat-file.
  -
  - Currently this always has to look in HEAD, because cat-file --batch
  - does not offer a way to specify that we want to look up a tree object