urlutil: add a `get_push_paths` to perform the push destination logic

urlutil: add a `get_push_paths` to perform the push destination logic

As is this changeset does not change anything. However having an official empty
point will help unifying the logic and encapsulate the details and update the
logic to support path definition pointing to multiple other path.

Differential Revision: https://phab.mercurial-scm.org/D10377

--- a/mercurial/commands.py	Mon Apr 12 06:34:54 2021 +0200
+++ b/mercurial/commands.py	Mon Apr 12 22:22:56 2021 +0200
@@ -5720,12 +5720,9 @@
                 # this lets simultaneous -r, -b options continue working
                 opts.setdefault(b'rev', []).append(b"null")
 
-    if not dests:
-        dests = [None]
     some_pushed = False
     result = 0
-    for dest in dests:
-        path = ui.getpath(dest, default=(b'default-push', b'default'))
+    for path in urlutil.get_push_paths(repo, ui, dests):
         if not path:
             raise error.ConfigError(
                 _(b'default repository not configured!'),

--- a/mercurial/utils/urlutil.py	Mon Apr 12 06:34:54 2021 +0200
+++ b/mercurial/utils/urlutil.py	Mon Apr 12 22:22:56 2021 +0200
@@ -445,6 +445,14 @@
     return bytes(u)
 
 
+def get_push_paths(repo, ui, dests):
+    """yields all the `path` selected as push destination by `dests`"""
+    if not dests:
+        dests = [None]
+    for dest in dests:
+        yield ui.getpath(dest, default=(b'default-push', b'default'))
+
+
 def parseurl(path, branches=None):
     '''parse url#branch, returning (url, (branch, branches))'''
     u = url(path)