rerere: lift PATH_MAX limitation

The MERGE_RR file records a collection of NUL-terminated entries,
each of which consists of

 - a hash that identifies the conflict
 - a HT
 - the pathname

We used to read this piece-by-piece, and worse yet, read the
pathname part a byte at a time into a fixed buffer of size PATH_MAX.

Instead, read a whole entry using strbuf_getwholeline() and parse
out the fields.  This way, we issue fewer read(2) calls and more
importantly we do not have to limit the pathname to PATH_MAX.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

rerere.c

@@ -35,32 +35,27 @@ static int has_rerere_resolution(const char *hex)
 
 static void read_rr(struct string_list *rr)
 {
-	unsigned char sha1[20];
-	char buf[PATH_MAX];
+	struct strbuf buf = STRBUF_INIT;
 	FILE *in = fopen(merge_rr_path, "r");
+
 	if (!in)
 		return;
-	while (fread(buf, 40, 1, in) == 1) {
-		int i;
-		char *name;
-		if (get_sha1_hex(buf, sha1))
+	while (!strbuf_getwholeline(&buf, in, '\0')) {
+		char *path;
+		unsigned char sha1[20];
+
+		/* There has to be the hash, tab, path and then NUL */
+		if (buf.len < 42 || get_sha1_hex(buf.buf, sha1))
 			die("corrupt MERGE_RR");
-		buf[40] = '\0';
-		name = xstrdup(buf);
-		if (fgetc(in) != '\t')
+
+		if (buf.buf[40] != '\t')
 			die("corrupt MERGE_RR");
-		for (i = 0; i < sizeof(buf); i++) {
-			int c = fgetc(in);
-			if (c < 0)
-				die("corrupt MERGE_RR");
-			buf[i] = c;
-			if (c == 0)
-				 break;
-		}
-		if (i == sizeof(buf))
-			die("filename too long");
-		string_list_insert(rr, buf)->util = name;
+		buf.buf[40] = '\0';
+		path = buf.buf + 41;
+
+		string_list_insert(rr, path)->util = xstrdup(buf.buf);
 	}
+	strbuf_release(&buf);
 	fclose(in);
 }