imap-send: simplify v_issue_imap_cmd() and get_cmd_result() using starts_with()

Use starts_with() instead of memcmp() to check if NUL-terminated
strings match prefixes.  This gets rid of some magic string length
constants.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

imap-send.c

@@ -526,7 +526,7 @@ static struct imap_cmd *v_issue_imap_cmd(struct imap_store *ctx,
 	if (Verbose) {
 		if (imap->num_in_progress)
 			printf("(%d in progress) ", imap->num_in_progress);
-		if (memcmp(cmd->cmd, "LOGIN", 5))
+		if (!starts_with(cmd->cmd, "LOGIN"))
 			printf(">>> %s", buf);
 		else
 			printf(">>> %d LOGIN <user> <pass>\n", cmd->tag);
@@ -829,7 +829,7 @@ static int get_cmd_result(struct imap_store *ctx, struct imap_cmd *tcmd)
 				} else /*if (!strcmp("BAD", arg))*/
 					resp = RESP_BAD;
 				fprintf(stderr, "IMAP command '%s' returned response (%s) - %s\n",
-					 memcmp(cmdp->cmd, "LOGIN", 5) ?
+					!starts_with(cmdp->cmd, "LOGIN") ?
 							cmdp->cmd : "LOGIN <user> <pass>",
 							arg, cmd ? cmd : "");
 			}