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