blame.c: Properly initialize strbuf after calling textconv_object(), again

2564aa4 started to initialize buf.alloc, but that should actually be one
more byte than the string length due to the trailing \0. Also, do not
modify buf.alloc out of the strbuf code. Use the existing strbuf_attach
instead.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/blame.c

@@ -2077,6 +2077,7 @@ static struct commit *fake_working_tree_commit(struct diff_options *opt,
 	if (!contents_from || strcmp("-", contents_from)) {
 		struct stat st;
 		const char *read_from;
+		char *buf_ptr;
 		unsigned long buf_len;
 
 		if (contents_from) {
@@ -2094,10 +2095,8 @@ static struct commit *fake_working_tree_commit(struct diff_options *opt,
 		switch (st.st_mode & S_IFMT) {
 		case S_IFREG:
 			if (DIFF_OPT_TST(opt, ALLOW_TEXTCONV) &&
-			    textconv_object(read_from, mode, null_sha1, &buf.buf, &buf_len)) {
-				buf.alloc = buf_len;
-				buf.len = buf_len;
-			}
+			    textconv_object(read_from, mode, null_sha1, &buf_ptr, &buf_len))
+				strbuf_attach(&buf, buf_ptr, buf_len, buf_len + 1);
 			else if (strbuf_read_file(&buf, read_from, st.st_size) != st.st_size)
 				die_errno("cannot open or read '%s'", read_from);
 			break;