clonebundle: add a `filter_bundle_url` function

This function does nothing by default, but give extension the opportunity to
alter the URL, typically, this could be used to inject authentication token when
serving clone bundle for private repositories.

mercurial/bundlecaches.py

@@ -25,8 +25,29 b' urlreq = util.urlreq'
 
 CB_MANIFEST_FILE = b'clonebundles.manifest'
 
+
 def get_manifest(repo):
-    return repo.vfs.tryread(CB_MANIFEST_FILE)
+    """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()]
+
+    new_lines = []
+    for e in entries:
+        url = alter_bundle_url(repo, e[0])
+        if len(e) == 1:
+            line = url + b'\n'
+        else:
+            line = b"%s %s\n" % (url, e[1])
+        new_lines.append(line)
+    return b''.join(new_lines)
+
+
+def alter_bundle_url(repo, url):
+    """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
+    of cached bundles."""
+    return url
 
 
 @attr.s