grep: use designated initializers for `grep_defaults`

grep: use designated initializers for `grep_defaults`

In 15fabd1bbd ("builtin/grep.c: make configuration callback more
reusable", 2012-10-09), we learned to fill a `static struct grep_opt
grep_defaults` which we can use as a blueprint for other such structs.

At the time, we didn't consider designated initializers to be widely
useable, but these days, we do. (See, e.g., cbc0f81d96 ("strbuf: use
designated initializers in STRBUF_INIT", 2017-07-10).)

Use designated initializers to let the compiler set up the struct and so
that we don't need to remember to call `init_grep_defaults()`.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>