Adds git history squash <revision-range> to fold a range of commits.

Changes in v5:

 * The range walk now uses --ancestry-path, so only commits descended from
   the base are folded; a single revision such as HEAD or HEAD~1 is now
   rejected as "not a <base>..<tip> range" rather than treated as a squash
   down to the root.
 * This adopts the --ancestry-path suggestion; the multi-base rejection is
   unchanged, so a side branch that forked before the base and merged in is
   still refused.
 * Added tests covering more merge topologies: two interior merges, a nested
   merge, an octopus merge, an octopus arm forked before the base, a merge
   among the descendants replayed above the range, and a ref pointing at an
   interior merge commit.

Changes in v4:

 * git history squash now detects when another ref points at a commit inside
   the range being folded and refuses, with an advice.historyUpdateRefs hint
   to use --update-refs=head.
 * A merge inside the range is folded fine as long as the range has a single
   base; a range with merge commit at the tip or base also folds correctly.
   Only a range with more than one base is rejected.

Changes in v3:

 * Moved the feature out of git rebase and into a new git history squash
   <revision-range> subcommand, per the list discussion. git rebase --squash
   is dropped.
 * Takes an arbitrary range (git history squash @~3.., git history squash
   @~5..@~2), folding it into the oldest commit and replaying any
   descendants on top.
 * Implemented as a single tree operation rather than picking each commit,
   so there are no repeated conflict stops (addresses Phillip's efficiency
   point).
 * A merge inside the range is folded fine, only a range with more than one
   base is rejected.
 * --reedit-message seeds the editor with every folded-in message, not just
   the oldest.

Harald Nordgren (4):
  history: extract helper for a commit's parent tree
  history: give commit_tree_ext a message template
  history: add squash subcommand to fold a range
  history: re-edit a squash with every message

 Documentation/config/advice.adoc |   4 +
 Documentation/git-history.adoc   |  26 ++
 advice.c                         |   1 +
 advice.h                         |   1 +
 builtin/history.c                | 341 ++++++++++++++++++---
 t/meson.build                    |   1 +
 t/t3455-history-squash.sh        | 497 +++++++++++++++++++++++++++++++
 7 files changed, 833 insertions(+), 38 deletions(-)
 create mode 100755 t/t3455-history-squash.sh

base-commit: 26d8d94e94df5535eecd036f16627493506a0614

Submitted-As: https://lore.kernel.org/git/pull.2337.v5.git.git.1782338102.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.2337.git.git.1781465141.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.2337.v2.git.git.1781512625.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.2337.v3.git.git.1781810226.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.2337.v4.git.git.1782021195.gitgitgadget@gmail.com