Add & use data_update method for Index.
Add & use data_update method for Index.

data_update is a convenience method for taking a python string and using it as
the contents of a given filename with a given mode in this index.

Signed-off-by: Chris Larson <8cf06b7089d5169434d5def8b2d1c9c9c95f6e71@mvista.com>

kergoth/git-origin