octopus-merge (part 3.5: gix-api and CLI)

[Byron]

Follow-up of #1585 .

Tasks

• blob configuration keys
• gix-API to merge three files
• gix merge-file

Next PR / Outscoped

• textconv with context, see this gist for details.
  • There seem to be different 'tiers' of tools, some don't get GIT_DIR set, others do.
  • It also seems that diff-programs get too much context right now, but that depends on how much is passed to them by the caller as gix-command::Context.
• How to model virtual-merge-bases? Can be none or many, user should have control over how this is done.
  • there is tests for that in diff3-conflict-markers
• Understand how conflicts are currently handled in GitButler.
• Actual tree-based merging
  • See also, all the merge options
  • Submodule merges are also possible! Maybe outscope it though! libgit2 also doesn't try it.
  • diff3-conflict-markers.sh - be sure to capture the 'empty tree' label , but also other special cases
• Make blob-merge based conflict handling possible in the tree merge from gix at least.

Research

Everything is about MergeORT.

• it uses an empty tree if there is no merge-base - we must allow the same.
• it allows for multiple merge-bases, creating a virtual one by merging all merge-bases together using the same algorithm, recursively.
• merges can have conflicts without a individual files being involved, for instance when directory renames clash
• Must make sure that possible types of conflicts are properly communicated, to not degenerate information
• It puts conflict-markers in the blobs of the result tree, with annotations to provide additional context
• Need resolution configuration, see git2::MergeOptions.
• data stored by path, and is interned in the map to allow pointer-based comparisons
  • merge-info with everything one needs to know, also related to renames
  • or conflict information
  • it uses a memory-pool/arena to get memory for many paths all at once (and also release it like that)
• paths start out as conflicted, and then can later be changed to non-conflicting if a content-based merged succeeded.
  • If it remains conflicts, the meta-data is used to produce an 'as merged as possible' version with conflict markers that can be checked out to the working tree.
• hunks can partially overlap, but can also be resolved line-by line to some extend.