convert trivial uses of strncmp() to skip_prefix()

As with the previous patch, using skip_prefix() is more readable and
less error-prone than a raw strncmp(), because it avoids a
manually-computed length. These cases differ from the previous patch
that uses starts_with() because they care about the value after the
matched prefix.

We can convert these to use skip_prefix() by introducing an extra
variable to hold the out-pointer.

Note in the case in ws.c that to get rid of the magic number "9"
completely, we also switch out "len" for recomputing the pointer
difference. These are equivalent because "len" is always "ep - string".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/remote-ext.c

@@ -169,6 +169,8 @@ static int command_loop(const char *child)
 
 	while (1) {
 		size_t i;
+		const char *arg;
+
 		if (!fgets(buffer, MAXCOMMAND - 1, stdin)) {
 			if (ferror(stdin))
 				die("Command input error");
@@ -182,10 +184,10 @@ static int command_loop(const char *child)
 		if (!strcmp(buffer, "capabilities")) {
 			printf("*connect\n\n");
 			fflush(stdout);
-		} else if (!strncmp(buffer, "connect ", 8)) {
+		} else if (skip_prefix(buffer, "connect ", &arg)) {
 			printf("\n");
 			fflush(stdout);
-			return run_child(child, buffer + 8);
+			return run_child(child, arg);
 		} else {
 			fprintf(stderr, "Bad command");
 			return 1;

ws.c

@@ -29,6 +29,7 @@ unsigned parse_whitespace_rule(const char *string)
 		int i;
 		size_t len;
 		const char *ep;
+		const char *arg;
 		int negated = 0;
 
 		string = string + strspn(string, ", \t\n\r");
@@ -52,15 +53,15 @@ unsigned parse_whitespace_rule(const char *string)
 				rule |= whitespace_rule_names[i].rule_bits;
 			break;
 		}
-		if (strncmp(string, "tabwidth=", 9) == 0) {
+		if (skip_prefix(string, "tabwidth=", &arg)) {
-			unsigned tabwidth = atoi(string + 9);
+			unsigned tabwidth = atoi(arg);
 			if (0 < tabwidth && tabwidth < 0100) {
 				rule &= ~WS_TAB_WIDTH_MASK;
 				rule |= tabwidth;
 			}
 			else
 				warning("tabwidth %.*s out of range",
-					(int)(len - 9), string + 9);
+					(int)(ep - arg), arg);
 		}
 		string = ep;
 	}