largefiles: simplify iteration over standins

Instead of iterating over all files in the context and ignoring those
that are not standins, pass a standin-matcher to the context and
iterate over only the files matching.

Apart from making the intent clearer, this implementation will also
benefit from any future optimizations done to the manifest walking
code.

hgext/largefiles/reposetup.py

@@ -193,9 +193,7 b' def reposetup(ui, repo):'
 
                     # Standins no longer found in lfdirstate has been
                     # removed
-                    for standin in ctx1.manifest():
-                        if not lfutil.isstandin(standin):
-                            continue
+                    for standin in ctx1.walk(lfutil.getstandinmatcher(self)):
                         lfile = lfutil.splitstandin(standin)
                         if not match(lfile):
                             continue