Re-fix rebase -i with SHA-1 collisions

Triggered by one of brian's SHA-256 patch series, I looked at the reason why the SHA-1 collision test case passed when it shouldn't. Turns out that the regression test was not quite thorough enough, and the interactive rebase did regress recently.

While in the area, I realized that the same bug exists in the code backing the rebase.missingCommitsCheck feature: the backed-up todo list uses shortened commit IDs that may very well become ambiguous during the rebase. For good measure, this patch series fixes that, too.

Finally, I saw that git rebase --edit-todo reported the line in an awkward, maybe even incorrect, way when there was an ambiguous commit ID, and I also fixed that.

To make sure that the code can be easily adapted to SHA-256 after these patches, I actually already made those adjustments on top and offered them up at https://github.com/bk2204/git/pull/1.

Changes since v2:

* Clarified in the first commit message that the change from a positive return value to a negative return value (to indicate failure, in both cases) both was intentional, and does not require any change in the corresponding function's only caller.

Changes since v1:

* Turned the error condition when parsing the todo list with just-expanded commit IDs failed into a BUG().

Johannes Schindelin (3):
parse_insn_line(): improve error message when parsing failed rebase -i: re-fix short SHA-1 collision
rebase -i: also avoid SHA-1 collisions with missingCommitsCheck

 rebase-interactive.c          |  8 +++++---
 sequencer.c                   | 18 ++++++++++++++----
 t/t3404-rebase-interactive.sh | 17 +++++++++++++++--
 3 files changed, 34 insertions(+), 9 deletions(-)

base-commit: d0654dc308b0ba76dd8ed7bbb33c8d8f7aacd783
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-529%2Fdscho%2Fre-fix-rebase-i-with-sha-collisions-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-529/dscho/re-fix-rebase-i-with-sha-collisions-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/529

Range-diff vs v2:

1: 2ae2e435b0 ! 1: 6f5c7b0325 parse_insn_line(): improve error message when parsing failed
@@ -7,6 +7,11 @@
That makes it rather hard for users to understand what is going wrong, so let's fix that.

+ While at it, return a negative value from parse_insn_line() in case of
+ an error, as per our convention. This function's only caller,
+ todo_list_parse_insn_buffer(), cares only whether that return value is
+ non-zero or not, i.e. does not need to be changed.
+

diff --git a/sequencer.c b/sequencer.c

 2:  102fa568dc = 2:  595ba81e09 rebase -i: re-fix short SHA-1 collision
 3:  486e9413a6 = 3:  b7b063408f rebase -i: also avoid SHA-1 collisions with missingCommitsCheck

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