builtin/blame.c: remove '--indent-heuristic' from usage string

builtin/blame.c: remove '--indent-heuristic' from usage string

The indent heuristic is our default diff heuristic since 33de716387
(diff: enable indent heuristic by default, 2017-05-08), but the usage
string of 'git blame' still mentions it as "experimental heuristic".

We could simply update the short help associated with the option, but
according to the comment above the option's declaration it was "only
included here to get included in the "-h" output".  That made sense
while the feature was still experimental and we wanted to give it more
exposure, but nowadays it's unnecessary.

So let's rather remove the '--indent-heuristic' option from 'git
blame's usage string.  Note that 'git blame' will still accept this
option, as it is parsed in parse_revision_opt().

Astute readers may notice that this patch removes a comment mentioning
"the following two options", but it only removes one option.  The
reason is that the comment is outdated: that other options was
'--compaction-heuristic', and it has already been removed in
3cde4e02ee (diff: retire "compaction" heuristics, 2016-12-23), but
that commit forgot to update this comment.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>