Merge branch 'tb/commit-graph-fd-exhaustion-fix'

Merge branch 'tb/commit-graph-fd-exhaustion-fix'

The commit-graph code exhausted file descriptors easily when it
does not have to.

* tb/commit-graph-fd-exhaustion-fix:
  commit-graph: close descriptors after mmap
  commit-graph.c: gracefully handle file descriptor exhaustion
  t/test-lib.sh: make ULIMIT_FILE_DESCRIPTORS available to tests
  commit-graph.c: don't use discarded graph_name in error