Rollup merge of rust-lang#63429 - rust-lang:gitignore-readd-tmp, r=Mark-Simulacrum

.gitignore: Readd `/tmp/`

Specifically, `/tmp/partitioning-tests/` it is generated by the incremental tests, https://github.com/rust-lang/rust/search?p=2&q=partitioning-tests&unscoped_q=partitioning-tests. These are cleaned up by compiletest but not if you kill testing prematurely (which I just did to test out a rollup, and it is annoying to `rm -rf tmp/`).

r? @Mark-Simulacrum
cc @RalfJung

Author: Centril

.gitignore

@@ -27,6 +27,7 @@ __pycache__/
 /inst/
 /llvm/
 /mingw-build/
+# Created by default with `src/ci/docker/run.sh`:
 /obj/
 /rustllvm/
 /src/libcore/unicode/DerivedCoreProperties.txt
@@ -38,6 +39,8 @@ __pycache__/
 /src/libcore/unicode/UnicodeData.txt
 /src/libcore/unicode/downloaded
 /target/
+# Generated by compiletest for incremental:
+/tmp/
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