subrepo: add test for Windows relative-ish path with drive letter

Matt Harbison pointed out that Windows had some weird path syntax.
Fortunately it's rejected appropriately by pathauditor, so we're safe.
Let's test the behavior as we have a special handling for Windows drive
letters.

This patch includes a basic example. Maybe we'll need to extend the test
case further, but writing such tests on Linux isn't easy.

tests/test-audit-subrepo.t

@@ -660,3 +660,44 b' then pull (and update):'
   $ cd ..
 
 #endif
+
+Test drive letter
+-----------------
+
+Windows has a weird relative path that can change the drive letter, which
+should also be prohibited on Windows.
+
+prepare tampered repo:
+
+  $ hg init driveletter
+  $ cd driveletter
+  $ hg import --bypass -qm 'add subrepo "X:"' - <<'EOF'
+  > diff --git a/.hgsub b/.hgsub
+  > new file mode 100644
+  > --- /dev/null
+  > +++ b/.hgsub
+  > @@ -0,0 +1,1 @@
+  > +X: = foo
+  > diff --git a/.hgsubstate b/.hgsubstate
+  > new file mode 100644
+  > --- /dev/null
+  > +++ b/.hgsubstate
+  > @@ -0,0 +1,1 @@
+  > +0000000000000000000000000000000000000000 X:
+  > EOF
+  $ cd ..
+
+on clone (and update):
+
+#if windows
+
+  $ hg clone -q driveletter driveletter2
+  abort: path contains illegal component: X:
+
+#else
+
+  $ hg clone -q driveletter driveletter2
+  $ ls driveletter2
+  X:
+
+#endif