test-support: Add [DIRTY-MSVC] to compare.rs docs

Author: dnbln

crates/cargo-test-support/src/compare.rs

@@ -16,6 +16,11 @@
 //!   `[WARNING]`) to match cargo's "status" output and allows you to ignore
 //!   the alignment. See the source of `substitute_macros` for a complete list
 //!   of substitutions.
+//! - `[DIRTY-MSVC]` (only when the line starts with it) would be replaced by
+//!   `[DIRTY]` when `cfg(target_env = "msvc")` or the line will be ignored otherwise.
+//!   Tests that work around [issue 7358](https://github.com/rust-lang/cargo/issues/7358)
+//!   can use this to avoid duplicating the `with_stderr` call like:
+//!   `if cfg!(target_env = "msvc") {e.with_stderr("...[DIRTY]...");} else {e.with_stderr("...");}`.
 //!
 //! # Normalization
 //!