Configuration for git to ignore generated test output on rebase/merge (#3773)

## Why
Having output file in conflicted state requires marking them as resolve
manually which adds to rebase/merge toil. You're going to regenerate
them anyway, so no need to spend time on these conflicts.

## Tests
Manually, by creating a conflicting change in script & output.txt:

```
% git rebase -i a
Auto-merging acceptance/selftest/basic/output.txt
Auto-merging acceptance/selftest/basic/script
CONFLICT (content): Merge conflict in acceptance/selftest/basic/script
error: could not apply 73b9af638... update Y
...
Could not apply 73b9af638... # update Y
```

Author: denik

.gitconfig

@@ -0,0 +1,5 @@
+[merge "ours"]
+    # define merge driver "ours" that will do nothing which will make git
+    # accept the file from the branch the we rebase onto as final version, ignoring local changes.
+    # https://git-scm.com/docs/gitattributes#_defining_a_custom_merge_driver
+    driver = true

.gitconfig.install

@@ -0,0 +1,3 @@
+#!/bin/bash
+set -ex
+git config --local include.path "$(git rev-parse --show-toplevel)/.gitconfig"

acceptance/.gitattributes

@@ -6,3 +6,7 @@
 
 # The out.test.toml file is autogenerated based on the merged test.toml view.
 out.test.toml linguist-generated=true
+
+# Do not mark generated files as conflicted, they should simply be regenerated.
+# Note, you need top run .gitconfig.install to enable 'ours' driver
+out* merge=ours