sha1_name: add get_sha1_with_context()

sha1_name: add get_sha1_with_context()

Textconv is defined by the diff driver, which is associated with a pathname,
not a blob. This fonction permits to know the context for the sha1 you're
looking for, especially his pathname

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>