Check some `.git` paths to force a rebuild

When a checked-out repo switches to a specific tag, and that tag does
not have anything else changed in the files except the tag, it could
happen that the build process doesn't see any changes, while it could be
that the version string needs to be different.

This commit ensures that if some specific paths are changed within the
.git directory, cargo will be triggered to rebuild.

Fixes #4087

build.rs

@@ -48,6 +48,11 @@ fn run(args: &[&str]) -> Result<String, std::io::Error> {
 ///    - env!("GIT_REV")
 ///    - env!("VW_VERSION")
 fn version_from_git_info() -> Result<String, std::io::Error> {
+    // Rerun when these paths are changed.
+    // Someone could have checked-out a tag or specific commit, but no other files changed.
+    println!("cargo:rerun-if-changed=.git/HEAD");
+    println!("cargo:rerun-if-changed=.git/refs/tags/");
+
     // The exact tag for the current commit, can be empty when
     // the current commit doesn't have an associated tag
     let exact_tag = run(&["git", "describe", "--abbrev=0", "--tags", "--exact-match"]).ok();