lfs: move the initialization of the upload request into the try block

This (almost) guarantees that the file is closed in the case of an exception.
The one hole is if the `seek(SEEK_END)`/`tell()`/`seek(0)` sequence fails.  But
that's going to go away when subclassing `httpconnection.httpsendfile` to fix
the worker problem, so I'm not going to worry too much.  (And that class appears
to have the same problem.)

Differential Revision: https://phab.mercurial-scm.org/D7959

hgext/lfs/blobstore.py

@@ -490,15 +490,17 b' class _gitlfsremote(object):'
                     _(b'detected corrupt lfs object: %s') % oid,
                     hint=_(b'run hg verify'),
                 )
-            request.data = lfsuploadfile(localstore.open(oid))
-            request.get_method = lambda: 'PUT'
-            request.add_header('Content-Type', 'application/octet-stream')
-            request.add_header('Content-Length', len(request.data))
 
         for k, v in headers:
             request.add_header(pycompat.strurl(k), pycompat.strurl(v))
 
         try:
+            if action == b'upload':
+                request.data = lfsuploadfile(localstore.open(oid))
+                request.get_method = lambda: 'PUT'
+                request.add_header('Content-Type', 'application/octet-stream')
+                request.add_header('Content-Length', len(request.data))
+
             with contextlib.closing(self.urlopener.open(request)) as res:
                 contentlength = res.info().get(b"content-length")
                 ui = self.ui  # Shorten debug lines