prefer "!=" when checking read_in_full() result

Comparing the result of read_in_full() using less-than is
potentially dangerous, as a negative return value may be
converted to an unsigned type and be considered a success.
This is discussed further in 561598cfcf (read_pack_header:
handle signed/unsigned comparison in read result,
2017-09-13).

Each of these instances is actually fine in practice:

 - in get-tar-commit-id, the HEADERSIZE macro expands to a
   signed integer. If it were switched to an unsigned type
   (e.g., a size_t), then it would be a bug.

 - the other two callers check for a short read only after
   handling a negative return separately. This is a fine
   practice, but we'd prefer to model "!=" as a general
   rule.

So all of these cases can be considered cleanups and not
actual bugfixes.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/get-tar-commit-id.c

@@ -26,7 +26,7 @@ int cmd_get_tar_commit_id(int argc, const char **argv, const char *prefix)
 		usage(builtin_get_tar_commit_id_usage);
 
 	n = read_in_full(0, buffer, HEADERSIZE);
-	if (n < HEADERSIZE)
+	if (n != HEADERSIZE)
 		die("git get-tar-commit-id: read error");
 	if (header->typeflag[0] != 'g')
 		return 1;

csum-file.c

@@ -19,7 +19,7 @@ static void flush(struct sha1file *f, const void *buf, unsigned int count)
 
 		if (ret < 0)
 			die_errno("%s: sha1 file read error", f->name);
-		if (ret < count)
+		if (ret != count)
 			die("%s: sha1 file truncated", f->name);
 		if (memcmp(buf, check_buffer, count))
 			die("sha1 file '%s' validation error", f->name);

pkt-line.c

@@ -258,7 +258,7 @@ static int get_packet_data(int fd, char **src_buf, size_t *src_size,
 	}
 
 	/* And complain if we didn't get enough bytes to satisfy the read. */
-	if (ret < size) {
+	if (ret != size) {
 		if (options & PACKET_READ_GENTLE_ON_EOF)
 			return -1;