diffcore-pickaxe: Add regcomp_or_die()

There's another regcomp code block coming in this function that needs
the same error handling. This function can help avoid duplicating
error handling code.

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

diffcore-pickaxe.c

@@ -198,6 +198,18 @@ static void pickaxe(struct diff_queue_struct *q, struct diff_options *o,
 	*q = outq;
 }
 
+static void regcomp_or_die(regex_t *regex, const char *needle, int cflags)
+{
+	int err = regcomp(regex, needle, cflags);
+	if (err) {
+		/* The POSIX.2 people are surely sick */
+		char errbuf[1024];
+		regerror(err, regex, errbuf, 1024);
+		regfree(regex);
+		die("invalid regex: %s", errbuf);
+	}
+}
+
 void diffcore_pickaxe(struct diff_options *o)
 {
 	const char *needle = o->pickaxe;
@@ -206,18 +218,10 @@ void diffcore_pickaxe(struct diff_options *o)
 	kwset_t kws = NULL;
 
 	if (opts & (DIFF_PICKAXE_REGEX | DIFF_PICKAXE_KIND_G)) {
-		int err;
 		int cflags = REG_EXTENDED | REG_NEWLINE;
 		if (DIFF_OPT_TST(o, PICKAXE_IGNORE_CASE))
 			cflags |= REG_ICASE;
-		err = regcomp(&regex, needle, cflags);
-		if (err) {
-			/* The POSIX.2 people are surely sick */
-			char errbuf[1024];
-			regerror(err, &regex, errbuf, 1024);
-			regfree(&regex);
-			die("invalid regex: %s", errbuf);
-		}
+		regcomp_or_die(&regex, needle, cflags);
 		regexp = &regex;
 	} else {
 		kws = kwsalloc(DIFF_OPT_TST(o, PICKAXE_IGNORE_CASE)