git - easy way to merge 3 trees without touching index or working tree

Question

I have a rather devilish idea in my head that requires me to run different merge operations where I do not want to touch the index nor the working tree. I know about git merge-tree that I think used to do what I am requesting but it has been obsoleted (at least the possibility of providing the 3 trees and getting the resulting tree id as output). Are there other current ways to do it?

Just to be clear:

• I do not want to mess up with histories.
• I want to provide the base tree and the 2 tip trees.
• And last but not least, I do not want to mess up with the index nor the working tree.

All I need as output is the resulting tree id if merge succeeds.... if there are conflicts it's ok that I get nothing.

Answer 1

While git merge-tree already improved with Git 2.37/2.38 , only Git 2.40 (Q1 2023) adds the missing part you seek: " merge-tree " learns a new --merge-base option.

See commit 4cc9eb3 (24 Nov 2022), and commit 501e3ba , commit 66265a6 (11 Nov 2022) by Kyle Zhao ( yefengzkk ) .
^{(Merged by Junio C Hamano -- gitster -- in commit 7576e51 , 14 Dec 2022)}

merge-tree.c : add --merge-base= option

^{Signed-off-by: Kyle Zhao
Signed-off-by: Taylor Blau}

This patch will give our callers more flexibility to use git merge-tree ^{( man )} , such as:

 git merge-tree --write-tree --merge-base=branch^ HEAD branch

This does a merge of HEAD and branch, but uses branch^ as the merge-base.

And the reason why using an option flag instead of a positional argument is to allow additional commits passed to merge-tree to be handled via an octopus merge in the future.

Note: Specifying multiple bases is currently not supported

And:

merge-tree.c : allow specifying the merge-base when --stdin is passed

^{Signed-off-by: Kyle Zhao
Signed-off-by: Taylor Blau}

The previous commit added a --merge-base option in order to allow using a specified merge-base for the merge.
Extend the input accepted by --stdin to also allow a specified merge-base with each merge requested.
For example:

 printf "<b3> -- <b1> <b2>" | git merge-tree --stdin

does a merge of b1 and b2 , and uses b3 as the merge-base.

git merge-tree now includes in its man page :

INPUT FORMAT

' git merge-tree --stdin ' input format is fully text based. Each line has this format:

 [<base-commit> -- ]<branch1> <branch2>

If one line is separated by -- , the string before the separator is used for specifying a merge-base for the merge and the string after the separator describes the branches to be merged.