When constructing views, metadata is available about the location of the
file in the view's reference branch. Allows incorporating parts of the
directory hierarchy in a view.
For example `git annex view tag=* podcasts/=*` makes a view in the form
tag/showname.
Performance impact: I benchmarked git annex view tag=* in the conference
proceedings repo to take 6.459s before this change, and 6.544s after.
FWIW, I considered making the syntax for this be podcasts/*, which might
be easier for the user to learn. However, I think it's not as good:
* The user has to then juggle two different syntaxes, and podcasts/* will
be expanded by the shell so they also need to quote it, while podcasts/=*
is unlikely to be expanded by the shell.
* It would allow for things like podcasts/*/* and *.mp3 which do not
map well into views.
This commit was sponsored by Aurélien Pinceaux.
