Add "get nearest cache entry for artifact"

[andreaskurth]

To cache partial builds in Memora, the get command needs to be able to optionally get the "nearest" entry instead of the entry for the current commit (which is not cached yet).

This would then enable memora get --nearest in CI scripts such as

if ! memora get compiler; then
  # no up-to-date build found
  if ! memora get --nearest compiler then
    # no nearest found, clean build
    make -C compiler all
  else
    # found nearest, do incremental build
    make -C compiler incremental
  fi
  memora insert compiler
fi

The "nearest commit" has to be defined more precisely (it might be especially tricky after merges?) -- but we might be able to say "the nearest commit is only defined if it is unique". If the nearest commit is not defined, memora get --nearest would fail.