non-standard `conflict-marker-size` is not respected

[ada4a]

Describe the bug
If a project has conflict-marker-size set to a non-standard value (say 15), Lazygit ignores it, and continues to look for the markers of the default size.

1. if there are conflicts of size 7 but not 15, the file is incorrectly marked as unmerged by Lazygit, preventing git rebase/merge --continue
2. if there are conflicts of size 15 but not 7, the conflicts are automatically skipped, leading to weird stuff in the following steps

To Reproduce
Steps to reproduce the behavior:

1. Set conflict-marker-size in e.g. ~/.gitattributes:

echo `conflict-marker-size=15` >> ~/.gitattributes

3. Create a file with standard-sized conflict markers:

pub fn main() {
    let a = "\
<<<<<<< LEFT
left
||||||| BASE
base
=======
right
>>>>>>> RIGHT
";
}

5. Create an unrelated conflict, e.g. by creating two branches where one renames a to b, and the other -- to c
6. Try to merge the two branches
7. The preview window will highlight the not-actually-a-conflict (the part with conflict markers of size 7), while ignoring the actual conflict that occured

Expected behavior
The actual conflict is highlighted, and the other one isn't.

Version info:

$ lazygit -v
commit=, build date=, build source=nix, version=0.46.0, os=linux, arch=amd64, git version=2.48.1
$ git -v
git version 2.48.1

Additional context
The default conflict marker size of 7 (<<<<<<<) has become a de-facto standard, and Lazygit expects it as well. But the ability to change it is still very much present, and helpful for projects that provide conflict-related functionality and e.g. have tests containing conflict examples. The latter is the case for Mergiraf, a custom Git merge driver we've been working on.

As for a possible fix:
We had this issue in Mergiraf as well (https://codeberg.org/mergiraf/mergiraf/pulls/136). But being a merge driver, we get the value passed by Git when invoked, so obtaining the value was relatively easy. I'm not sure whether Lazygit can do something similar, but another option would be to just use git check-attr like this:

git check-attr conflict-marker-size <file_name>

After that, one could change

lazygit/pkg/gui/mergeconflicts/find_conflicts.go

Line 61 in 19ac926

CONFLICT_START = "<<<<<<< "

(and the other related constants) to instead be constructed based on the value.

[stefanhaller]

Hi Ada (if you're still around), sorry for the long delay reacting to this. I had to run into the problem myself to realize how bad it is. 😅

(I hadn't realized that there's a real reason to set the marker size; I somehow assumed people would set it for aesthetic reasons. I now ran into it when resolving a conflict in git.git, which sets the marker size for its test files which often contain "here" docs with literal conflict markers.)

Anyway, here's a fix if you feel like testing it: #5902

Oh btw, happy user of Mergiraf here, I use it often and like it.