Update shell_cmds.cpp

[maxpat78]

Converts 'do switch' in 'do if'

#Fixes #4531 introduced by internationalization changes.

Following sample batch now works.

@echo on
if not exist ADIR md ADIR
cd ADIR
echo 1 >1.txt
echo 2 >2.txt
echo 3 >3.txt
rem del /p ?.txt
del /p *.*
cd ..