Fix clang-format configuration

The configuration was only including files that were in a commit diff.
This causes issues where we generate sources during the PRESUBMIT (which
are not in the git diff at that point in time) and don't format.

At the same time, we remove the complicated logic to reuse the
`.eslintignore` for clang-format purposes and use the correct
`.clang-format` "DisableFormat" options to disable formatting in various
directories.

Roll CodeMirror: ignore

Change-Id: I21b95a6e09c5d6bf036bf1aace103d6c26823ecc
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2091445
Reviewed-by: Changhao Han <[email protected]>
Commit-Queue: Tim van der Lippe <[email protected]>
diff --git a/front_end/diff/.clang-format b/front_end/diff/.clang-format
new file mode 100644
index 0000000..37f3d57
--- /dev/null
+++ b/front_end/diff/.clang-format
@@ -0,0 +1 @@
+DisableFormat: true
\ No newline at end of file