textconv: support for cat_file

textconv: support for cat_file

Make the textconv_object function public, and add --textconv option to cat-file
to perform conversion on blob objects. Using --textconv implies that we are
working on a blob.
As files drivers need to be initialized, a new config is required in addition
to git_default_config. Therefore git_cat_file_config() is introduced

Signed-off-by: Clément Poulain <clement.poulain@ensimag.imag.fr>
Signed-off-by: Diane Gasselin <diane.gasselin@ensimag.imag.fr>
Signed-off-by: Axel Bonnet <axel.bonnet@ensimag.imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>