typing: add some trivial type hints to `mercurial/bundlecaches.py`

The function is meant for extensions, but it wasn't obvious what was expected
without looking through the code.  Also, pytype couldn't figure it out either.

mercurial/bundlecaches.py

@@ -30,7 +30,7 b" CB_MANIFEST_FILE = b'clonebundles.manife"
 CLONEBUNDLESCHEME = b"peer-bundle-cache://"
 
 
-def get_manifest(repo):
+def get_manifest(repo) -> bytes:
     """get the bundle manifest to be served to a client from a server"""
     raw_text = repo.vfs.tryread(CB_MANIFEST_FILE)
     entries = [e.split(b' ', 1) for e in raw_text.splitlines()]
@@ -46,7 +46,7 b' def get_manifest(repo):'
     return b''.join(new_lines)
 
 
-def alter_bundle_url(repo, url):
+def alter_bundle_url(repo, url: bytes) -> bytes:
     """a function that exist to help extension and hosting to alter the url
 
     This will typically be used to inject authentication information in the url