patch-id: use starts_with() and skip_prefix()

index 366ce5a..a84d000 100644
@@ -81,16 +81,13 @@ static int get_one_patchid(struct object_id *next_oid, struct object_id *result,
 
 	while (strbuf_getwholeline(line_buf, stdin, '\n') != EOF) {
 		char *line = line_buf->buf;
-		char *p = line;
+		const char *p = line;
 		int len;
 
-		if (!memcmp(line, "diff-tree ", 10))
-			p += 10;
-		else if (!memcmp(line, "commit ", 7))
-			p += 7;
-		else if (!memcmp(line, "From ", 5))
-			p += 5;
-		else if (!memcmp(line, "\\ ", 2) && 12 < strlen(line))
+		if (!skip_prefix(line, "diff-tree ", &p) &&
+		    !skip_prefix(line, "commit ", &p) &&
+		    !skip_prefix(line, "From ", &p) &&
+		    starts_with(line, "\\ ") && 12 < strlen(line))
 			continue;
 
 		if (!get_oid_hex(p, next_oid)) {

@@ -99,14 +96,14 @@ static int get_one_patchid(struct object_id *next_oid, struct object_id *result,
 		}
 
 		/* Ignore commit comments */
-		if (!patchlen && memcmp(line, "diff ", 5))
+		if (!patchlen && !starts_with(line, "diff "))
 			continue;
 
 		/* Parsing diff header?  */
 		if (before == -1) {
-			if (!memcmp(line, "index ", 6))
+			if (starts_with(line, "index "))
 				continue;
-			else if (!memcmp(line, "--- ", 4))
+			else if (starts_with(line, "--- "))
 				before = after = 1;
 			else if (!isalpha(line[0]))
 				break;

@@ -114,14 +111,14 @@ static int get_one_patchid(struct object_id *next_oid, struct object_id *result,
 
 		/* Looking for a valid hunk header?  */
 		if (before == 0 && after == 0) {
-			if (!memcmp(line, "@@ -", 4)) {
+			if (starts_with(line, "@@ -")) {
 				/* Parse next hunk, but ignore line numbers.  */
 				scan_hunk_header(line, &before, &after);
 				continue;
 			}
 
 			/* Split at the end of the patch.  */
-			if (memcmp(line, "diff ", 5))
+			if (!starts_with(line, "diff "))
 				break;
 
 			/* Else we're parsing another header.  */