largefiles: fix the directory structure when archiving a subrepo in a subrepo

Previously, a repo consisting of main/sub/subsub archived sub and subsub as
siblings under main.

hgext/largefiles/overrides.py

@@ -850,7 +850,7 b' def hgsubrepoarchive(orig, repo, ui, arc'
 
     for subpath in ctx.substate:
         sub = ctx.sub(subpath)
-        sub.archive(ui, archiver, prefix)
+        sub.archive(ui, archiver, os.path.join(prefix, repo._path) + '/')
 
 # If a largefile is modified, the change is not reflected in its
 # standin until a commit. cmdutil.bailifchanged() raises an exception

tests/test-subrepo-deep-nested-change.t

@@ -125,8 +125,8 b' Note that add --large through a subrepo '
   ../archive_lf/sub1/.hgsub
   ../archive_lf/sub1/.hgsubstate
   ../archive_lf/sub1/sub1
-  ../archive_lf/sub2
-  ../archive_lf/sub2/large.bin
-  ../archive_lf/sub2/sub2
+  ../archive_lf/sub1/sub2
+  ../archive_lf/sub1/sub2/large.bin
+  ../archive_lf/sub1/sub2/sub2
 
   $ cd ..