streamclone: use readexactly when reading stream v2

Yuya Nishihara pointed out that it is safer.

mercurial/streamclone.py

@@ -594,12 +594,12 b' def consumev2(repo, fp, filecount, files'
                     for vfs in vfsmap.values())
             with nested(*ctxs):
                 for i in range(filecount):
-                    src = fp.read(1)
+                    src = util.readexactly(fp, 1)
                     vfs = vfsmap[src]
                     namelen = util.uvarintdecodestream(fp)
                     datalen = util.uvarintdecodestream(fp)
 
-                    name = fp.read(namelen)
+                    name = util.readexactly(fp, namelen)
 
                     if repo.ui.debugflag:
                         repo.ui.debug('adding [%s] %s (%s)\n' %