Close fd when aborting a merge

[icristescu]

When a merge is aborted, the fd for the merge file is not closed.

[icristescu]

Reopening because #326 only fixes the issue for the merge file. But as pointed out by @mattiasdrp in #326 (comment), the same issue also occurs for the log_async file: the file descriptor is not closed in case of an aborted merge.