run-make: Allow blessing snapshot files that don't exist yet

[Zalathar]

This makes it possible to bless the snapshot files used by diff() in newly-created run-make tests, without having to create the files manually beforehand.

r? jieyouxu

[rustbot]

jieyouxu is currently at their maximum review capacity.
They may take a while to respond.

[rustbot]

The run-make-support library was changed

cc @jieyouxu

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

##[endgroup]
[TIMING] core::build_steps::tool::ToolBuild { build_compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu, tool: "tidy", path: "src/tools/tidy", mode: ToolBootstrap, source_type: InTree, extra_features: [], allow_features: "", cargo_args: [], artifact_kind: Binary } -- 12.996
[TIMING] core::build_steps::tool::Tidy { compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu } -- 0.000
fmt check
Diff in /checkout/src/tools/run-make-support/src/diff/mod.rs:48:
         let path = path.as_ref();
         // In `--bless` mode, create the snapshot file if it doesn't already exist.
         // The empty file will be overwritten with the actual text.
-        if self.bless_dir.is_some() && let Ok(false) = std::fs::exists(path) {
+        if self.bless_dir.is_some()
+            && let Ok(false) = std::fs::exists(path)
+        {
             fs::write(path, "");
         }
         let content = fs::read_to_string(path);
fmt: checked 6293 files
Build completed unsuccessfully in 0:00:47
  local time: Tue Aug  5 12:42:58 UTC 2025
  network time: Tue, 05 Aug 2025 12:42:59 GMT
##[error]Process completed with exit code 1.

[jieyouxu]

Thanks, r=me after fixing tidy

[jieyouxu]

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.