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

diff --cc commit-graph.c
Simple merge

diff --cc t/t1400-update-ref.sh
Simple merge

diff --cc t/t5324-split-commit-graph.sh
Simple merge

diff --cc t/test-lib.sh
Simple merge