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.

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