Rollup merge of #65653 - RalfJung:gitignore, r=Mark-Simulacrum,Centril

keep the root dir clean from debugging

We landed this before with https://github.com/rust-lang/rust/pull/63307 but recently in https://github.com/rust-lang/rust/pull/65630 the IMO bad ignore crept back in.

If you regularly do graphviz-based debugging and you are fine leaving junk in the rustc root dir, please configure your local `.git/info/exclude`. But most people working on rustc don't work with graphciz all that often (I for once never did), and not everyone likes to have stray generated files in their source dirs.

Also Cc https://github.com/rust-lang/rust/pull/63373 https://github.com/rust-lang/rust/pull/53768 @ecstatic-morse @Mark-Simulacrum

.gitignore

@@ -1,6 +1,10 @@
-# This file should only ignore things that are generated during a build,
-# generated by common IDEs, and optional files controlled by the user
-# that affect the build (such as config.toml).
+# This file should only ignore things that are generated during a `x.py` build,
+# generated by common IDEs, and optional files controlled by the user that
+# affect the build (such as config.toml).
+# In particular, things like `mir_dump` should not be listed here; they are only
+# created during manual debugging and many people like to clean up instead of
+# having git ignore such leftovers. You can use `.git/info/exclude` to
+# configure your local ignore list.
 # FIXME: This needs cleanup.
 *~
 .#*
@@ -52,6 +56,4 @@ config.stamp
 Session.vim
 .cargo
 no_llvm_build
-# Generated when dumping Graphviz output for debugging:
-/mir_dump/
-/*.dot
+# Before adding new lines, see the comment at the top.