Add an --explain option that displays human readable descriptions of why
git-annex decides whether or not to transfer or drop a file. Eg:

    git-annex get bar foo --explain
    [bar is not present here]
    [foo is present here]
    get bar

    git-get get --auto --explain baz meep
    [baz is not present here]
    (baz has 1 copy, and its numcopies is 2)
    get baz
    [meep is not present here]
    [meep matches preferred content: include==* and exclude==archive/*]
    get meep

    git-annex drop --explain archive/foo
    [archive/foo is present here]
    [archive/foo has 2 copies, and its numcopies is 1]
    [archive/foo does not march preferred content: include==* and exclude!=archive/*]
    drop foo (checking origin...) ok

Implementation would be to output these extra lines when checking if
content is present, and when checking if preferred content expressions
match.

It could also display the actions taken while attempting to drop a file,
and the drop safety proof that is constructed. --debug already contains
that, but it could be made more human readable and included in --explain.

It should be possible, while matching a preferred content expression, to
accumulate a version of it with "=" replaced by "==" and "!=" depending on
whether a term matches. That seems clear enough to understand why it
matched. What to do about terms like "present"? Maybe "!present" when it
doesn't match?" And a term like "standard" would ideally expand out the
included expression, eg: "standard(group==backup, include==*)" 

[[!tag confirmed]] --[[Joey]]

[[!tag projects/dandi]]

> [[done]] --[[Joey]]