Allow merge commits when 'in-rust-tree' feature is enabled

This adds an `in-rust-tree` feature that will be enabled when
rust-analyzer is built from `rust-lang/rust`. Due to the way
"git subtree" works, that test _will_ find merge commits and
fail, so we simply skip it.

Author: fasterthanlime

crates/rust-analyzer/Cargo.toml

@@ -84,3 +84,4 @@ mbe = { path = "../mbe" }
 [features]
 jemalloc = ["jemallocator", "profile/jemalloc"]
 force-always-assert = ["always-assert/force"]
+in-rust-tree = []

crates/rust-analyzer/tests/slow-tests/tidy.rs

@@ -142,6 +142,7 @@ fn check_cargo_toml(path: &Path, text: String) {
     }
 }
 
+#[cfg(not(feature = "in-rust-tree"))]
 #[test]
 fn check_merge_commits() {
     let sh = &Shell::new().unwrap();