fork-point: add --explain flag

[PawelLipski]

Prints, alongside the fork point hash, the branches whose filtered reflog
the algorithm matched the commit against - the same set that
status --list-commits reports next to a -> fork point ??? annotation
on a yellow edge.

When a fork point override is active the override short-circuits the
reflog scan, so the list is empty and (none) is printed.

The flag composes with --inferred but is mutually exclusive with the
override-* / --unset-override flags.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.78%. Comparing base (6058a32) to head (efb78eb).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1672   +/-   ##
========================================
  Coverage    98.78%   98.78%           
========================================
  Files           42       42           
  Lines         5170     5187   +17     
  Branches       939      944    +5     
========================================
+ Hits          5107     5124   +17     
  Misses          39       39           
  Partials        24       24           

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