Naturally, prep_temp_blob() did not care about filenames.
As a result, GIT_EXTERNAL_DIFF and textconv generated
filenames such as ".diff_XXXXXX".
This modifies prep_temp_blob() to generate user-friendly
filenames when creating temporary files.
Diffing "name.ext" now generates "XXXXXX_name.ext".
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>