diffcore-pickaxe: remove fill_one()

diffcore-pickaxe: remove fill_one()

fill_one is _almost_ identical to just calling fill_textconv; the
exception is that for the !DIFF_FILE_VALID case, fill_textconv gives us
an empty buffer rather than a NULL one. Since we currently use the NULL
pointer as a signal that the file is not present on one side of the
diff, we must now switch to using DIFF_FILE_VALID to make the same
check.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Simon Ruderich <redacted>
Signed-off-by: Junio C Hamano <redacted>