introduce --merge to bring up merge editor (for #5770)

[bpasero]

This PR is for #5770

[bpasero]

@lramos15 @jrieken this is ready for a first look and should already work, despite not being the full solution yet, since the editor resolver is not yet working as we discussed, but I still think we could merge this to get feedback from users and work on the editor resolver independently.

To try it out:

• scripts/code-cli.sh --help will explain the new --merge option
• you have to pass in 4 paths and add --merge to bring up a merge editor

For Jo: mainly MergeEditorResolverContribution and friends where the merge editor factory gets registered. This is not yet the final look, once the resolver works as discussed I would only expect a single method being registered only for the merge editor.

For Logan: mainly editor resolver changes: there is currently a hack that I explicitly set the override: 'mergeEditor.Input' when opening the new untyped merge editor input. As discussed, it would be great if the resolver could find the editor by probing on the factory methods.

PS: at first I used names such as local, remote for the sides of the merge editor but then went back again to input1 and input2 like the merge editor internally uses. This makes the solution less opinionated around Git and more generic. The CLI also reflects this and pretty much aligns with this solution. The downside is that the labels for both sides are even more generic ("First Version", "Second Version").

//cc @hediet

[hediet]

The downside is that the labels for both sides are even more generic ("First Version", "Second Version").

Can we pass the labels for the sides as command line argument?

code -m $REMOTE $LOCAL $BASE $OUT --labelInput1 "Yours" --labelInput2 "Theirs"

[bpasero]

Can we pass the labels for the sides as command line argument?

Yeah, maybe in another PR, will keep a note. Maybe there is also some cool ways in git config to let Git fill in a nice label via some script tricks.

[lramos15]

👍 , I will start on the resolver work

[numandev1]

@bpasero @hediet thanks for working on this. I would say, 3 way merge UX is not soo much good. it should improve. as far as I know, IntelliJ merge UX is soo much mature