lei add-external --mirror: deduce paths for PSGI mount prefixes
The current manifest.js.gz generation in WWW doesn't account for
PSGI mount prefixes (and grokmirror 1.x appears to work fine).
In other words, <https://yhbt.net/lore/lkml/manifest.js.gz>
currently has keys like "/lkml/git/0.git" and not
"/lore/lkml/git/0.git" where "/lore" is the PSGI mount prefix.
This works fine with the prefix accounted for in my grokmirror
(1.x) repos.conf like this:
site = https://yhbt.net/lore/
manifest = https://yhbt.net/lore/manifest.js.gz
Adding the PSGI mount prefix in manifest.js.gz is probably not
desirable since it would force the prefix into the locally
cloned path by grokmirror, and all the cloned directories
would have the remote PSGI mount prefix prepended to the
toplevel.
So, "lei add-external --mirror" needs to account for PSGI
mount prefixes by deducing the prefix based on available keys
in the manifest.js.gz hash table.