Motivation

Resolves #5047

Implement the logic found in git-merge-octopus.sh as originally implemented by Junio C Hamano.

Changes

• merge.c
  • git_merge now supports merging multiple branches using the octopus strategy
  • add compute_base_octopus to verify an initial base exists
  • add git_merge__octopus_fastforward to perform the possible fastforward for the first branch in an octopus
  • add git_merge__octopus_simple to perform the simple merge of each branch in an octopus
  • add merge_annotated_commits_octopus as the octopus variant of merge_annotated_commits

Tests

• All tests use the tests/resources/merge-octopus repository, and start with t as the HEAD branch
• Simple case:
  • merge 4 unrelated branches into a target branch
  • verify expected index post-merge
  • command: git merge branch1 branch1-b branch2 branch3
• Failure case:
  • merge 3 unrelated branches with conflicts
  • verify merge fails with GIT_EMERGECONFLICT aka -24
  • command: git merge f1 f2 f3
• Complex case:
  • merge several branches with complex history
  • verify expected index post-merge
  • command: git merge 1 1a 1b 2 2a 3 t1 t2
• Fastforward and Skip case:
  • merge several branches, with the first admitting a fastforward and the second being skippable
  • verify expected index post-merge
  • command: git merge ff skippable 1

Miscellania

This is my first contribution! It was a lot of fun to work through this and see how the project works. Any and all feedback is appreciated!

I am also not sure I credited the original author correctly, so please take a look at that.

PS, sorry for triggering so many CI build actions; next time I will open the PR once I am no longer making as many pushes.