mktree: plug per-tree leak in --batch mode

In --batch mode "git mktree" reuses its entry buffer across trees,
resetting `used` to 0 after writing each tree.  It never frees the
`treeent` structures the previous tree appended, though, so once the
next tree overwrites those slots the earlier allocations are leaked.  A
single-tree invocation hides this, as the entries stay reachable through
the `entries` global until exit.

Free each entry when resetting the buffer, and free the buffer itself
before returning.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/mktree.c

@@ -200,8 +200,11 @@ int cmd_mktree(int ac,
 			puts(oid_to_hex(&oid));
 			fflush(stdout);
 		}
+		for (int i = 0; i < used; i++)
+			free(entries[i]);
 		used=0; /* reset tree entry buffer for re-use in batch mode */
 	}
+	free(entries);
 	strbuf_release(&sb);
 
 	return 0;