lfs: infer the blob store URL from an explicit pull source

I don't see any easier way to do this because the update part of `hg pull -u`
happens outside exchange.pull(), and commands.postincoming() doesn't take a
path.  So (ab)use the mechanism used by subrepos to redirect where subrepos are
pulled from when an explicit path is given.  As a bonus, this should allow lfs
blobs to be pulled into a subrepo when it is checked out.

An explicit push path can be handled within exchange.push().  That can be done
next, outside of this dirty hack.

hgext/lfs/blobstore.py

@@ -541,9 +541,15 b' def remote(repo):'
     """
     url = util.url(repo.ui.config('lfs', 'url') or '')
     if url.scheme is None:
-        # TODO: investigate 'paths.remote:lfsurl' style path customization,
-        # and fall back to inferring from 'paths.remote' if unspecified.
-        defaulturl = util.url(repo.ui.config('paths', 'default') or b'')
+        if util.safehasattr(repo, '_subtoppath'):
+            # The pull command sets this during the optional update phase, which
+            # tells exactly where the pull originated, whether 'paths.default'
+            # or explicit.
+            defaulturl = util.url(repo._subtoppath)
+        else:
+            # TODO: investigate 'paths.remote:lfsurl' style path customization,
+            # and fall back to inferring from 'paths.remote' if unspecified.
+            defaulturl = util.url(repo.ui.config('paths', 'default') or b'')
 
         # TODO: support local paths as well.
         # TODO: consider the ssh -> https transformation that git applies

tests/test-lfs-serve.t

@@ -253,7 +253,26 b' lfs content, and the extension enabled.'
   $TESTTMP/server/.hg/requires:lfs
 
   $ hg init $TESTTMP/client6_pull
-  $ hg -R $TESTTMP/client6_pull pull -q http://localhost:$HGPORT
+  $ hg -R $TESTTMP/client6_pull pull -u -v http://localhost:$HGPORT
+  pulling from http://localhost:$HGPORT/
+  requesting all changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 6 changesets with 5 changes to 5 files (+1 heads)
+  calling hook pretxnchangegroup.lfs: hgext.lfs.checkrequireslfs
+  new changesets d437e1d24fbd:d3b84d50eacb
+  resolving manifests
+  lfs: assuming remote store: http://localhost:$HGPORT/.git/info/lfs
+  lfs: downloading a82f1c5cea0d40e3bb3a849686bb4e6ae47ca27e614de55c1ed0325698ef68de (25 bytes)
+  lfs: processed: a82f1c5cea0d40e3bb3a849686bb4e6ae47ca27e614de55c1ed0325698ef68de
+  getting lfs2.txt
+  lfs: found a82f1c5cea0d40e3bb3a849686bb4e6ae47ca27e614de55c1ed0325698ef68de in the local lfs store
+  getting nonlfs2.txt
+  getting nonlfs3.txt
+  3 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  updated to "d3b84d50eacb: lfs file with lfs client"
+  1 other heads for branch "default"
   $ grep 'lfs' $TESTTMP/client6_pull/.hg/requires $SERVER_REQUIRES
   $TESTTMP/client6_pull/.hg/requires:lfs
   $TESTTMP/server/.hg/requires:lfs