BUG #13813: pg_upgrade fails to run pg_ctl

The following bug has been logged on the website:

Bug reference: 13813
Logged by: Roman D
Email address: romandashkov(at)gmail(dot)com
PostgreSQL version: 9.4.5
Operating system: Windows 10 x64
Description:

I ran pg_upgrade as a test, trying to move data files from 9.4.5 to 9.4.5
(same version in two separate directories). Both old and new servers were
off. Command issued:

pg_upgrade --username postgres --no-paswword -old-datadir... -new-datadir...
-old-bindir... -new-bindir...

It failed with this a message like this:

could not start old postmaster... with this command...
"path\to\data-directory/pg_ctl" -option 1 -option 2...

I noticed it is using a forward slash as a path to pg_ctl within the data
directory. It should be using backward slash in windows.

I'm sorry I was not able to record the error message exactly before it was
lost (and the logs too).