How does git figure out unmerged paths in conflicts?

How does git get the list of unmerged paths?

My understanding of git is that when merging it incorporates changes into a file and then adds it to index. If a merge conflict occurs, a file is not added to the index and is left in the working tree with conflict markers. If I run git status it shows me unmerged paths for the conflicted files.

How does git get the list of these files? If I run git add for a conflicted file with markers still in place, it will not be shown as conflicted anymore, so perhaps markers are not identifiers?

Solution

If merge conflict occurs a file is not added in index and left in working tree with conflict markers.

It is in the index: see git ls-files:

For an unmerged path, instead of recording a single mode/SHA-1 pair, the index records up to three such pairs; one from tree O in stage 1, A in stage 2, and B in stage 3.

git read-tree details the 2 way merge:

Each "index" entry has two bits worth of "stage" state.
stage 0 is the normal one, and is the only one you’d see in any kind of normal use.

However, when you do git read-tree with three trees, the "stage" starts out at 1.

This means that you can do

git read-tree -m <tree1> <tree2> <tree3>

and you will end up with an index with all of the <tree1> entries in "stage1", all of the <tree2> entries in "stage2" and all of the <tree3> entries in "stage3".
When performing a merge of another branch into the current branch, we use the common ancestor tree as <tree1>, the current branch head as <tree2>, and the other branch head as <tree3>.