subrepo: introduce a `repo_rel_or_abs_source` function

The `subrepoutil` module has various function to compute the path of a
sub-repository compared to the root of the top repository. However, they was no
function available to compute the relative path of the repository "source". And
we need this information for exchange operation (in our case, `hg outgoing`).
The information is currently build using the `repo._subtoppath` hack. We reuse
the same logic but in a dedicated function independent of the `repo._subtoppath`
hack.

Differential Revision: https://phab.mercurial-scm.org/D10434

mercurial/subrepoutil.py

@@ -383,6 +383,24 b' def precommit(ui, wctx, status, match, f'
     return subs, commitsubs, newstate
 
 
+def repo_rel_or_abs_source(repo):
+    """return the source of this repo
+
+    Either absolute or relative the outermost repo"""
+    parent = repo
+    chunks = []
+    while util.safehasattr(parent, b'_subparent'):
+        source = urlutil.url(parent._subsource)
+        chunks.append(bytes(source))
+        if source.isabs():
+            break
+        parent = parent._subparent
+
+    chunks.reverse()
+    path = posixpath.join(*chunks)
+    return posixpath.normpath(path)
+
+
 def reporelpath(repo):
     # type: (localrepo.localrepository) -> bytes
     """return path to this (sub)repo as seen from outermost repo"""