archive: support 'wdir()'...
archive: support 'wdir()'
This is a step toward replacing the extdiff internals with archive, in order to
support 'extdiff -S'. Only Mercurial subrepos are supported for now.
If a file is missing from the filesystem, it is silently skipped. Perhaps it
should warn, but it cannot abort when working with extdiff because deleting a
file is a legitimate diff.
