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>

parse-options-cb.c

@@ -16,6 +16,9 @@ int parse_opt_abbrev_cb(const struct option *opt, const char *arg, int unset)
 	if (!arg) {
 		v = unset ? 0 : DEFAULT_ABBREV;
 	} else {
+		if (!*arg)
+			return error(_("option `%s' expects a numerical value"),
+				     opt->long_name);
 		v = strtol(arg, (char **)&arg, 10);
 		if (*arg)
 			return error(_("option `%s' expects a numerical value"),

parse-options.c

@@ -193,6 +193,9 @@ static enum parse_opt_result get_value(struct parse_opt_ctx_t *p,
 		}
 		if (get_arg(p, opt, flags, &arg))
 			return -1;
+		if (!*arg)
+			return error(_("%s expects a numerical value"),
+				     optname(opt, flags));
 		*(int *)opt->value = strtol(arg, (char **)&s, 10);
 		if (*s)
 			return error(_("%s expects a numerical value"),