The python_bin_reldir.txt path to find the python2 binary will
contain backslashes instead of forward slashes on Windows, so
this tweaks the wrapper in //python2-bin/python2 to handle that
and start working under msys.

Bug: 1168365
Change-Id: If0876054f0a73c9a53a1eb23ea03c60c1e756c29
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2729410
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>