rebase: learn --keep-base and improvements on fast-forward behaviour

Thanks for your suggestions, Pratyush and Philip. I've incorporated both of them into this reroll.

Changes since v1:

* Squashed old set into one patch
* Fixed indentation style and dangling else
* Added more documentation after discussion with Ævar

Changes since v2:

* Add testing for rebase --fork-point behaviour
* Add testing for rebase fast-forward behaviour
* Make rebase --onto fast-forward in more cases
* Update documentation to include use-case

Changes since v3:

* Fix tests failing on bash 4.2
* Fix typo in t3431 comment

Changes since v4:

* Make rebase --fork-point fast-forward in more cases

Changes since v5:

* Fix graph illustrations so that the "branch off" is visually in the correct place
* Refactor if-else in can_fast_forward into one-level-deep ifs to increase clarity

Changes since v6:

* Remove redundant braces around if
* Update comment around can_fast_forward call
* Add completion for rebase

Changes since v7:

* Ævar sent in his WIP patchset

Changes since v8:

* Drop patches 9-13
* Fix some minor whitespace issues from v7

Changes since v9:

* Remove unnecessary if-statement in patch 6/9
* Update commit message for patch 9/9 for clarity according to Philip's suggestions

[1]: https://public-inbox.org/git/a1b4b74b9167e279dae4cd8c58fb28d8a714a66a.1553537656.git.gitgitgadget@gmail.com/

Denton Liu (6):
t3431: add rebase --fork-point tests
t3432: test rebase fast-forward behavior
rebase: refactor can_fast_forward into goto tower
rebase: fast-forward --onto in more cases
rebase: fast-forward --fork-point in more cases
rebase: teach rebase --keep-base

Ævar Arnfjörð Bjarmason (3):
t3432: distinguish "noop-same" v.s. "work-same" in "same head" tests
t3432: test for --no-ff's interaction with fast-forward rebase tests: test linear branch topology

 Documentation/git-rebase.txt           |  30 +++++-
 builtin/rebase.c                       |  84 ++++++++++++-----
 contrib/completion/git-completion.bash |   2 +-
 t/t3400-rebase.sh                      |   2 +-
 t/t3404-rebase-interactive.sh          |   2 +-
 t/t3416-rebase-onto-threedots.sh       |  57 +++++++++++
 t/t3421-rebase-topology-linear.sh      |  29 ++++++
 t/t3431-rebase-fork-point.sh           |  57 +++++++++++
 t/t3432-rebase-fast-forward.sh         | 125 +++++++++++++++++++++++++
 9 files changed, 360 insertions(+), 28 deletions(-)
 create mode 100755 t/t3431-rebase-fork-point.sh
 create mode 100755 t/t3432-rebase-fast-forward.sh

Range-diff against v9:

 1:  03f769d410 =  1:  03f769d410 t3431: add rebase --fork-point tests
 2:  bc8998079d =  2:  bc8998079d t3432: test rebase fast-forward behavior
 3:  5c08e2b81f =  3:  5c08e2b81f t3432: distinguish "noop-same" v.s. "work-same" in "same head" tests
 4:  48b4e41a17 =  4:  48b4e41a17 t3432: test for --no-ff's interaction with fast-forward
 5:  9bd34b4a40 !  5:  9acce7c911 rebase: refactor can_fast_forward into goto tower
    @@ builtin/rebase.c: static int can_fast_forward(struct commit *onto, struct object
     -		res = 0;
     +		goto done;
      	}
    --	free_commit_list(merge_bases);
     +
     +	oidcpy(merge_base, &merge_bases->item->object.oid);
     +	if (!oideq(merge_base, &onto->object.oid))
    @@ builtin/rebase.c: static int can_fast_forward(struct commit *onto, struct object
     +	res = 1;
     +
     +done:
    -+	if (merge_bases)
    -+		free_commit_list(merge_bases);
    + 	free_commit_list(merge_bases);
      	return res && is_linear_history(onto, head);
      }
    - 
 6:  becb924232 =  6:  3f208421d6 rebase: fast-forward --onto in more cases
 7:  4dab5847cb =  7:  126c20a95d rebase: fast-forward --fork-point in more cases
 8:  4699a90993 =  8:  bf6cc6a610 rebase tests: test linear branch topology
 9:  6927aba617 !  9:  6bc7423ac1 rebase: teach rebase --keep-base

@@ Commit message

in order to preserve the merge base. This is useful when contributing a patch series to the Git mailing list, one often starts on top of the
- current 'master'. However, while developing the patches, 'master' is
- also developed further and it is sometimes not the best idea to keep
- rebasing on top of 'master', but to keep the base commit as-is.
+ current 'master'. While developing the patches, 'master' is also
+ developed further and it is sometimes not the best idea to keep rebasing
+ on top of 'master', but to keep the base commit as-is.

- Alternatively, a user wishing to test individual commits in a topic
- branch without changing anything may run
+ In addition to this, a user wishing to test individual commits in a
+ topic branch without changing anything may run

git rebase -x ./test.sh master... master

2.23.0.248.g3a9dd8fb08