diff: spell DIFF_INDEX_CACHED out when calling run_diff_index()

diff: spell DIFF_INDEX_CACHED out when calling run_diff_index()

Many callers of run_diff_index() passed literal "1" for the option
flag word, which should better be spelled out as DIFF_INDEX_CACHED
for readablity.  Everybody else passes "0" that can stay as-is.

The other bit in the option flag word is DIFF_INDEX_MERGE_BASE, but
curiously there is only one caller that can pass it, which is "git
diff-index --merge-base" itself---no internal callers uses the
feature.

A bit tricky call to the function is in builtin/submodule--helper.c
where the .cached member in a private struct is set/reset as a plain
Boolean flag, which happens to be "1" and happens to match the value
of DIFF_INDEX_CACHED.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>