FR: add additional information after conflict markers

[poliorcetics]

Is your feature request related to a problem? Please describe.

Example of a conflict currently:

<<<<<<<
%%%%%%%
-        let git: Box<dyn DiffProvider> = Box::new(Git);
-        let providers = vec![git];
+        let providers = vec![Git.into()];
+++++++
        #[cfg(feature = "git")]
        let git: Box<dyn DiffProvider> = Box::new(git::Git);
        let providers = vec![
            #[cfg(feature = "git")]
            git,
        ];
>>>>>>>

The issue here is that I don't remember where I was coming from since it's been some time since I last touched this.

Describe the solution you'd like

Adding some more information allowing to know what comes from where and to look for information could be very useful.

Suggestion:

<<<<<<< base: wuurqlntozxqr (main); conflict: uvponskzxlpzot
%%%%%%% diff: wuurqlntozxqr (-) to uvponskzxlpzot (+)
-        let git: Box<dyn DiffProvider> = Box::new(Git);
-        let providers = vec![git];
+        let providers = vec![Git.into()];
+++++++ added in uvponskzxlpzot
        #[cfg(feature = "git")]
        let git: Box<dyn DiffProvider> = Box::new(git::Git);
        let providers = vec![
            #[cfg(feature = "git")]
            git,
        ];
>>>>>>>

In practice something of the form:

<<<<<<< base: CHANGE_ID_BASE (branch names, if any) ; conflict: CHANGE_ID_CONFLICT
%%%%%%% diff: CHANGE_ID_BASE (-) to CHANGE_ID_CONFLICT (+)
------- removed in CHANGE_ID_X
+++++++ added in CHANGE_ID_X

Describe alternatives you've considered

1. Do nothing: this works but information is missing that could be useful to understand conflict markers

Additional context

Git does something similar:

$ git init \
  && git branch -m main \
  && echo -e "Foo\nHello" > file.txt \
  && git add file.txt \
  && git commit -m "a" \
  && git branch new-branch \
  && git switch new-branch \
  && echo -e "Bar\nFarwell" > file.txt \
  && git add file.txt \
  && git commit -m "b" \
  && git switch main \
  && echo -e "Bar\nHello" > file.txt \
  && git add file.txt \
  && git commit -m "c" \
  && git merge new-branch

$ cat file.txt

Bar
<<<<<<< HEAD
Hello
||||||| 492f92b
Foo
Hello
=======
Farwell
>>>>>>> new-branch

Here this is a merge so:

• HEAD is the local change
• 492f92b is the common ancestor
• new-branch is the remote change being merged

[ilyagr]

I'm actually working on a simpler version of this, see #1176 (comment). I think this is a duplicate of that bug; there are some difficulties you can read about in the discussion over there.