debugfs: display the tested path and mount point of the filesystem, if known

While implementing win32.getfstype(), I noticed that MSYS path mangling is
getting in the way.  Given a path \\host\share\dir:

  - If strong quoted, hg receives it unchanged, and it works as expected
  - If double quoted, it converts to \host\share\dir
  - If unquoted, it converts to \hostsharedir

The second and third cases are problematic because those are valid paths
relative to the current drive letter, so os.path.realpath() will expand it as
such.  The net effect is to silently turn a network path test into (typically) a
"C:\" test.  Additionally, the command hangs after printing out 'symlink: no'
for the third case (but is interruptable with Ctrl + C).  This path mangling
only comes into play because of the command line arguments- it won't affect
internally obtained paths.  Therefore, the simplest thing to do is to provide
feedback on what the command is acting on.

I also added the mount point, because Windows supports nesting [1] volumes (see
the examples in "Junction Points and Mounted Folders"), and it was a useful
diagnostic for figuring out why the wrong filesystem was printed out in the
cases above.

I opted not to call os.path.realpath() on the path argument, to make it clearer
that the mangling isn't being done by Mercurial.

[1] https://msdn.microsoft.com/en-us/library/windows/desktop/aa364996(v=vs.85).aspx

mercurial/debugcommands.py

@@ -911,6 +911,8 b' def debugformat(ui, repo, **opts):'
 @command('debugfsinfo', [], _('[PATH]'), norepo=True)
 def debugfsinfo(ui, path="."):
     """show information detected about current filesystem"""
+    ui.write(('path: %s\n') % path)
+    ui.write(('mounted on: %s\n') % (util.getfsmountpoint(path) or '(unknown)'))
     ui.write(('exec: %s\n') % (util.checkexec(path) and 'yes' or 'no'))
     ui.write(('fstype: %s\n') % (util.getfstype(path) or '(unknown)'))
     ui.write(('symlink: %s\n') % (util.checklink(path) and 'yes' or 'no'))