Improve config.toml vs config symlink check on Unix

The code attempts to suppress the warning if `config` is merely a
symlink to `config.toml`.  But it does this by calling readlink and
comparing the result textually.

That's not ideal because it depends on the precise spelling
(`config.toml` vs `./config.toml` vs `/path/to/config.toml`)
and also minds whether it's config.toml -> config, or v.v.
On Unix, where we can get st_dev and st_ino, use that to suppress the
check when the two names resolve to the same underlying file.

src/cargo/util/context/mod.rs

@@ -1545,6 +1545,29 @@ impl GlobalContext {
                 // WITH an extension, which people may want to do to
                 // support multiple Cargo versions at once and not
                 // get a warning.
+
+                // On Unix we can see if the two files are the same by using stat(2),
+                // and comparing the inode numbers.  That way it doesn't matter precisely
+                // what spelling the link uses to refers to the other file (relative
+                // vs absolute pathname, etc.)  Also, this is symmetrical: it doesn't mind
+                // which way round the symlink is.
+                #[cfg(unix)]
+                let skip_warning = if let (Ok(metadata_1), Ok(metadata_2)) = (
+                    fs::metadata(&possible),
+                    fs::metadata(&possible_with_extension),
+                ) {
+                    use std::os::unix::fs::MetadataExt;
+                    let devino = |md: fs::Metadata| (md.dev(), md.ino());
+                    devino(metadata_1) == devino(metadata_2)
+                } else {
+                    false
+                };
+
+                // Platforms other than unix don't have std::os::unix::fs::MetadataExt
+                // and its st_ino and st_dev methods.  We can still ignore a symlink,
+                // but only based on its *contents*, which we must hope are identical
+                // to our computed pathname.
+                #[cfg(not(unix))]
                 let skip_warning = if let Ok(target_path) = fs::read_link(&possible) {
                     target_path == possible_with_extension ||
                         // allow `config` -> `config.toml`, without the parent dir(s),