rust-chg: add brief comment about initial capacity of temp_sock_path()

I don't know if it can be expressed as a compile-time constant, so it's
a comment for now.

About this series:
This is quite old patches for rust-chg. I heard from Octobus people that
there's a plan to do an experiment on merging hgcli + chg + some Rust?,
so I decided to respin the rust-chg series.

Maybe we'll rewrite the core to leverage the recent async/await functionality,
but I want to first make my old patches in so the rust-chg can be a drop-in
replacement for the chg of C. Compiler warnings will be removed later, and
the codebase will be upgraded to the 2018 edition later.

Differential Revision: https://phab.mercurial-scm.org/D8357

rust/chg/src/locator.rs

@@ -47,7 +47,7 b' impl Locator {'
     /// Temporary socket path for this client process.
     fn temp_sock_path(&self) -> PathBuf {
         let src = self.base_sock_path.as_os_str().as_bytes();
-        let mut buf = Vec::with_capacity(src.len() + 6);
+        let mut buf = Vec::with_capacity(src.len() + 6); // "{src}.{pid}".len()
         buf.extend_from_slice(src);
         buf.extend_from_slice(format!(".{}", self.process_id).as_bytes());
         OsString::from_vec(buf).into()