So the user can now switch to a view and then move files around within it
to manage metadata. For example, moving a file into a new directory
when in the tags=* view adds a tag to it.
Implementation is fairly efficient. One diff-index, which is no more
expensive than the first stage of a git commit, followed by possibly
some cat-file --batch traffic to find the key (when deleting a file).
Very similar to what's done in direct mode when committing. And like
direct mode when updating the WC after a merge, it has to buffer the
diff-tree values in order to make 2 passes over them.
When not in a view, pre-commit now does one extra git symbolic-ref,
which is tiny overhead.
This commit was sponsored by Andrew Eskridge.
Adds metadata log, and command.
Note that unsetting field values seems to currently be broken.
And in general this has had all of 2 minutes worth of testing.
This commit was sponsored by Julien Lefrique.
