parse-options: check empty value in OPT_INTEGER and OPT_ABBREV

parse-options: check empty value in OPT_INTEGER and OPT_ABBREV

When parsing the argument for OPT_INTEGER and OPT_ABBREV, we check if we
can parse the entire argument to a number with "if (*s)". There is one
missing check: if "arg" is empty to begin with, we fail to notice.

This could happen with long option by writing like

  git diff --inter-hunk-context= blah blah

Before 16ed6c97cc (diff-parseopt: convert --inter-hunk-context,
2019-03-24), --inter-hunk-context is handled by a custom parser
opt_arg() and does detect this correctly.

This restores the bahvior for --inter-hunk-context and make sure all
other integer options are handled the same (sane) way. For OPT_ABBREV
this is new behavior. But it makes it consistent with the rest.

PS. OPT_MAGNITUDE has similar code but git_parse_ulong() does detect
empty "arg". So it's good to go.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>