mcf: Fix MCF-PATCHES for branch with slash

:Release Notes:
There are other dangerous characters which could be used in branch
names and have special meaning in shell, but for now we have use-case
only for slash in our branch naming convention.

:Detailed Notes:
Fixes errors like this when called mcf --command update
when some branch with slash is defined in weboslayers.py:
2013-12-05T16:45:48 DEBUG __main__ git format-patch
  origin/jansa/dylan-backports..jansa/dylan-backports
  -o ./MCF-PATCHES_jansa/dylan-backports-20131206004548
fatal: Could not create directory
  './MCF-PATCHES_jansa/dylan-backports-20131206004548': No such file
  or directory

:Testing Performed:
"mcf --command update" now works

:QA Notes:

:Issues Addressed:
[GF-54399] Creating MCF-PATCHES directory fails if selected branch
           contains '/' or other special chars for shell

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>
Change-Id: I5daa37794aaede784cdf6ef4fb7d18b196b38ea9
Reviewed-on: https://g2g.palm.com/4302
Reviewed-by: DCO Verification
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Tested-by: Martin Jansa <Martin.Jansa@lge.com>
Build: Susan Montooth <susan.montooth@lge.com>
Reviewed-by: Build Verification
Reviewed-by: Susan Montooth <susan.montooth@lge.com>
Tested-by: Susan Montooth <susan.montooth@lge.com>