commit-graph: suggest deleting corrupt graphs

At $WORK, we've had a few occasions where someone's commit-graph becomes corrupt, and hits various BUG()s that block their day-to-day work. When this happens, we advise the user to either disable the commit graph, or to delete it and let it be regenerated.

It would be a nicer user experience if we can make this a self-serve procedure. To do this, let's add a new git commit-graph clear subcommand so that users don't need to manually delete files under their .git directories. And to make it self-documenting, update various BUG(), die(), and error() messages to suggest removing the commit graph to recover from the corruption.

This approach was suggested in [1] and generally positively received [2], but was never implemented.

[1] https://lore.kernel.org/git/[email protected]/
[2] https://lore.kernel.org/git/[email protected]/

Open questions for reviewers:
* Should we turn this into an advice setting instead?
* Should we also suggest running commit-graph write after clearing the graph? I lean towards no; everything will still function as normal without a commit graph.
* Does it make sense to add the suggestion in all of these corruption error messages? There are many other error()s in commit-graph.c, should we add this for all of them, or just the ones that specifically mention corruption? Or maybe just the fatal BUG()s and die()s?
* Any other places this suggestion should be added that I've missed?

Josh Steadmon (2):
commit-graph: add git commit-graph clear subcommand
commit-graph: suggest removing corrupt graphs

 Documentation/git-commit-graph.txt |  5 ++++
 builtin/commit-graph.c             | 40 +++++++++++++++++++++++++++
 commit-graph.c                     | 43 +++++++++++++++++++++++++++---
 commit-graph.h                     |  1 +
 commit-reach.c                     |  4 ++-
 t/t5318-commit-graph.sh            | 17 ++++++++++--
 t/t5324-split-commit-graph.sh      | 26 ++++++++++++------
 7 files changed, 122 insertions(+), 14 deletions(-)

base-commit: 3e0d3cd5c7def4808247caf168e17f2bbf47892b

2.44.0.rc0.258.g7320e95886-goog