grep API: call grep_config() after grep_init()

grep API: call grep_config() after grep_init()

The grep_init() function used the odd pattern of initializing the
passed-in "struct grep_opt" with a statically defined "grep_defaults"
struct, which would be modified in-place when we invoked
grep_config().

So we effectively (b) initialized config, (a) then defaults, (c)
followed by user options. Usually those are ordered as "a", "b" and
"c" instead.

As the comments being removed here show the previous behavior needed
to be carefully explained as we'd potentially share the populated
configuration among different instances of grep_init(). In practice we
didn't do that, but now that it can't be a concern anymore let's
remove those comments.

This does not change the behavior of any of the configuration
variables or options. That would have been the case if we didn't move
around the grep_config() call in "builtin/log.c". But now that we call
"grep_config" after "git_log_config" and "git_format_config" we'll
need to pass in the already initialized "struct grep_opt *".

See 6ba9bb76e02 (grep: copy struct in one fell swoop, 2020-11-29) and
7687a0541e0 (grep: move the configuration parsing logic to grep.[ch],
2012-10-09) for the commits that added the comments.

The memcpy() pattern here will be optimized away and follows the
convention of other *_init() functions. See 5726a6b4012 (*.c *_init():
define in terms of corresponding *_INIT macro, 2021-07-01).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>