Code cleanup with more careful error checking before using data
read from the commit-graph file.

* ab/commit-graph-fixes:
  commit-graph: improve & i18n error messages
  commit-graph write: don't die if the existing graph is corrupt
  commit-graph verify: detect inability to read the graph
  commit-graph: don't pass filename to load_commit_graph_one_fd_st()
  commit-graph: don't early exit(1) on e.g. "git status"
  commit-graph: fix segfault on e.g. "git status"
  commit-graph tests: test a graph that's too small
  commit-graph tests: split up corrupt_graph_and_verify()