Tweak "conflict markers" diagnostic by detecting git state

[estebank]

When detecting "diff markers", inspect the current git repo's state to see if there is an active rebase. If that is the case, we try to identify whether the rebase is caused by git rebase or implicitly by git pull. In order to do that we query git to ask for the path of .git/rebase-merge and then inspect the files orig-head (the commit we are rebasing from), onto (the commit we are rebasing onto) and head-name (the branch being rebased). We modify the labels pointing at the markers to be clear on where each section of code is coming from.

git rebase case:

error: encountered diff marker
 --> src/main.rs:2:1
  |
2 | <<<<<<< HEAD
  | ^^^^^^^ between this marker and `=======` is the code from branch `master` we're rebasing onto
3 |     println!("Hello, main!");
4 | =======
  | ------- between this marker and `>>>>>>>` is the code you had in branch `branch-2` that you are rebasing
5 |     println!("Hello, branch!");
6 | >>>>>>> e644375 (branch)
  | ^^^^^^^ this marker concludes the conflict region
  |
  = note: conflict markers indicate that a merge was started but could not be completed due to merge conflicts
          to resolve a conflict, keep only the code you want and then delete the lines containing conflict markers
  = note: for an explanation on these markers from the `git` documentation:
          visit <https://git-scm.com/book/en/v2/Git-Tools-Advanced-Merging#_checking_out_conflicts>

git pull case:

error: encountered diff marker
 --> src/main.rs:2:1
  |
2 | <<<<<<< HEAD
  | ^^^^^^^ between this marker and `=======` is the code from the remote branch `origin/main` we're rebasing onto
3 |     println!("Hello, 1!");
4 | =======
  | ------- between this marker and `>>>>>>>` is the code you had in branch `main` that you are rebasing
5 |     println!("Hello, 2!");
6 | >>>>>>> ebbeec7 (second)
  | ^^^^^^^ this marker concludes the conflict region
  |
  = note: conflict markers indicate that a merge was started but could not be completed due to merge conflicts
          to resolve a conflict, keep only the code you want and then delete the lines containing conflict markers
  = note: for an explanation on these markers from the `git` documentation:
          visit <https://git-scm.com/book/en/v2/Git-Tools-Advanced-Merging#_checking_out_conflicts>

Also, the generic help description had the git pull case flipped from what it should have been.

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[estebank]

I'll push a run-make test for the different cases being handled shortly.

This is what it looks like in my two test repos:

git rebase case:

git pull case:

and the current output:

[estebank]

Given the logic above this might need to be more generic, calling it a branch instead of a remote branch, or reference the commit sha...