idf_tools.py: pass --work-tree instead of -C option to git
idf_tools.py: pass --work-tree instead of -C option to git

Older versions of git do not support -C option. Use --work-tree option
instead.

Closes https://github.com/espressif/esp-idf/issues/4018
Merges https://github.com/espressif/esp-idf/pull/4019

espressif/esp-idf,espressif/esp-idf,espressif/esp-idf,espressif/esp-idf