read_gitfile_gently(): rename misnamed function to read_gitfile()

read_gitfile_gently(): rename misnamed function to read_gitfile()

The function was not gentle at all to the callers and died without giving
them a chance to deal with possible errors. Rename it to read_gitfile(),
and update all the callers.

As no existing caller needs a true "gently" variant, we do not bother
adding one at this point.

Signed-off-by: Junio C Hamano <gitster@pobox.com>