subrepo: normalize path in the specific way for problematic encodings

Before this patch, "reporelpath()" uses "rstrip(os.sep)" to trim
"os.sep" at the end of "parent.root" path.

But it doesn't work correctly with some problematic encodings on
Windows, because some multi-byte characters in such encodings contain
'\\' (0x5c) as the tail byte of them.

In such cases, "reporelpath()" leaves unexpected '\\' at the beginning
of the path returned to callers.

"lcalrepository.root" seems not to have tail "os.sep", because it is
always normalized by "os.path.realpath()" in "vfs.__init__()", but in
fact it has tail "os.sep", if it is a root (of the drive): path
normalization trims tail "os.sep" off "/foo/bar/", but doesn't trim
one off "/".

So, just avoiding "rstrip(os.sep)" in "reporelpath()" causes
regression around issue3033 fixed by fccd350acf79.

This patch introduces "pathutil.normasprefix" to normalize specified
path in the specific way for problematic encodings without regression
around issue3033.

mercurial/pathutil.py

@@ -142,3 +142,25 b' def canonpath(root, cwd, myname, auditor'
             name = dirname
 
         raise util.Abort(_("%s not under root '%s'") % (myname, root))
+
+def normasprefix(path):
+    '''normalize the specified path as path prefix
+
+    Returned vaule can be used safely for "p.startswith(prefix)",
+    "p[len(prefix):]", and so on.
+
+    For efficiency, this expects "path" argument to be already
+    normalized by "os.path.normpath", "os.path.realpath", and so on.
+
+    See also issue3033 for detail about need of this function.
+
+    >>> normasprefix('/foo/bar').replace(os.sep, '/')
+    '/foo/bar/'
+    >>> normasprefix('/').replace(os.sep, '/')
+    '/'
+    '''
+    d, p = os.path.splitdrive(path)
+    if len(p) != len(os.sep):
+        return path + os.sep
+    else:
+        return path

mercurial/subrepo.py

@@ -276,8 +276,7 b' def reporelpath(repo):'
     parent = repo
     while util.safehasattr(parent, '_subparent'):
         parent = parent._subparent
-    p = parent.root.rstrip(os.sep)
-    return repo.root[len(p) + 1:]
+    return repo.root[len(pathutil.normasprefix(parent.root)):]
 
 def subrelpath(sub):
     """return path to this subrepo as seen from outermost repo"""

tests/test-doctest.py

@@ -19,6 +19,7 b" testmod('mercurial.hg')"
 testmod('mercurial.hgweb.hgwebdir_mod')
 testmod('mercurial.match')
 testmod('mercurial.minirst')
+testmod('mercurial.pathutil')
 testmod('mercurial.revset')
 testmod('mercurial.store')
 testmod('mercurial.subrepo')