rust: Preallocate the returned `Vec` in `utils::files::relativize_path` Profiling `rhg files > /dev/null` on an old snapshot of mozilla-central (with `perf` and the Firefox Profiler: https://github.com/firefox-devtools/profiler/blob/main/docs-user/guide-perf-profiling.md) showed non-trivial time spend in this function and in `realloc`. This change makes the wall-clock time for that process on my machine go from ~190 ms to ~150 ms. Differential Revision: https://phab.mercurial-scm.org/D10199