Prevent --index-info from ignoring -z.

Prevent --index-info from ignoring -z.

If git-update-index --index-info -z is used only the first
record given to the process will actually be updated as
the -z option is ignored until after all index records
have been read and processed.  This meant that multiple
null terminated records were seen as a single record which
was lacking a trailing LF, however since the first record
ended in a null the C string handling functions ignored the
trailing garbage.  So --index-info should be required to be
the last command line option, much as --stdin is required
to be the last command line option.  Because --index-info
implies --stdin this isn't an issue as the user shouldn't
be passing --stdin when also passing --index-info.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>