Document that the accepted variants of the --track option are --track,
--track=direct, and --track=inherit.  The equal sign in the latter two
cannot be replaced with whitespace; in general optional arguments need
to be attached firmly to their option.

Put "direct" consistently before "inherit", if only for the reasons
that the former is the default, explained first in the documentation,
and comes before the latter alphabetically.

Mention both modes in the short help so that readers don't have to look
them up in the full documentation.  They are literal strings and thus
untranslatable.  PARSE_OPT_LITERAL_ARGHELP is inferred due to the pipe
and parenthesis characters, so we don't have to provide that flag
explicitly.

Mention that -t has the same effect as --track and --track=direct.
There is no way to specify inherit mode using the short option, because
short options generally don't accept optional arguments.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>