[Feature request] autoresolve modify/delete conflict via .gitattributes merge= attribute

[ilya Basin <basinilya@gmail.com>]

Hi. This is also described in this SO question:
https://stackoverflow.com/questions/58767528/how-to-specify-a-git-merge-ours-strategy-with-gitattributes-for-deleted-files

Our proprietary IDE generates theme-cache/web/theme.compiled.css and the contents are different on different machines.
This file was supposed to be in .gitignore, but it slipped into the repo.
I was trying to remove the file, but other developers silently re-introduce it into the repo. Here's what happens:
- The proprietary IDE re-generates the file and makes a local commit.
- The IDE does "pull --rebase" which results in a "deleted by us" conflict
- The IDE immediately re-generates the file, clearing the conflicted state
- Even though it's in .gitignore, the file remains under version control because of the cherry-pick by rebase

I was hoping that adding a certain merge= attribute for this file would help, but apparently it only considered when two files are merged, not when one is deleted.