commit-graph: close descriptors after mmap

commit-graph: close descriptors after mmap

We don't ever refer to the descriptor after mmap-ing it. And keeping it
open means we can run out of descriptors in degenerate cases (e.g.,
thousands of split chain files). Let's close it as soon as possible.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>