build: emit warning if git is missing during build

Closes #3057

build.rs

@@ -37,10 +37,25 @@ fn set_git_revision_hash() {
     use std::process::Command;
 
     let args = &["rev-parse", "--short=10", "HEAD"];
-    let Ok(output) = Command::new("git").args(args).output() else { return };
-    let rev = String::from_utf8_lossy(&output.stdout).trim().to_string();
+    let output = Command::new("git").args(args).output();
+    match output {
+        Ok(output) => {
+            let rev =
+                String::from_utf8_lossy(&output.stdout).trim().to_string();
             if rev.is_empty() {
+                println!(
+                    "cargo:warning=output from `git rev-parse` is empty, \
+                     so skipping embedding of commit hash"
+                );
                 return;
             }
             println!("cargo:rustc-env=RIPGREP_BUILD_GIT_HASH={rev}");
         }
+        Err(e) => {
+            println!(
+                "cargo:warning=failed to run `git rev-parse`, \
+                 so skipping embedding of commit hash: {e}"
+            );
+        }
+    }
+}