[PATCH v2] upload-pack commit graph segfault fix

Here's a re-roll of my "disable commit graph more gently" fix. Versus v1:

  - I've included a preparatory patch that speeds up
    prepare_commit_graph(). It's not strictly related, but there's a
    textual dependency. It could be easily spun off to its own series.

  - a comment points out that the ordering of the "disable" check is
    important

  - disable_commit_graph() now works on a repository struct

  - typo fixes in the test comments

  [1/2]: commit-graph: bump DIE_ON_LOAD check to actual load-time
  [2/2]: upload-pack: disable commit graph more gently for shallow traversal

 commit-graph.c        | 18 +++++++++++++++---
 commit-graph.h        |  6 ++++++
 repository.h          |  3 +++
 t/t5500-fetch-pack.sh | 38 ++++++++++++++++++++++++++++++++++++++
 upload-pack.c         |  2 +-
 5 files changed, 63 insertions(+), 4 deletions(-)

-Peff