Add a Cargo build script that sets LIBLOOT_REVISION

It only does so if git is on the PATH and `git rev-parse --short HEAD` produces a UTF-8 string, but the script should never fail.

README.md

@@ -31,7 +31,9 @@ libloot-<last tag>-<revisions since tag>-g<short revision ID>_<branch>-<platform
 
 Make sure you have [Rust](https://www.rust-lang.org/) installed.
 
-The `LIBLOOT_REVISION` environment variable is used to embed the commit hash into the build. If it's not defined then `unknown` will be used instead. To define it in PowerShell, run:
+The `LIBLOOT_REVISION` environment variable is used to embed the commit hash into the build. If it's not defined then `unknown` will be used instead. The Cargo build script will automatically define it if libloot is built from a Git repository and `git` is accessible from your `PATH`.
+
+To define it in PowerShell, run:
 
 ```powershell
 $env:LIBLOOT_REVISION = git rev-parse --short HEAD

build.rs

@@ -0,0 +1,38 @@
+use std::process::Command;
+
+fn main() {
+    if std::env::var_os("LIBLOOT_REVISION").is_some() {
+        return;
+    }
+
+    println!("cargo:rerun-if-changed=.git/HEAD");
+    println!("cargo:rerun-if-changed=build.rs");
+
+    let Ok(git_rev_parse_output) = Command::new("git")
+        .args(["rev-parse", "--short", "HEAD"])
+        .output()
+    else {
+        println!("cargo:warning=Calling git rev-parse failed, LIBLOOT_REVISION will be unset!");
+        return;
+    };
+
+    if let Ok(git_hash) = String::from_utf8(git_rev_parse_output.stdout) {
+        println!("cargo:rustc-env=LIBLOOT_REVISION={git_hash}");
+    } else {
+        println!(
+            "cargo:warning=Could not convert git rev-parse output to a UTF-8 string, LIBLOOT_REVISION will be unset!"
+        );
+    }
+
+    // Don't warn if this errors, as it will error if HEAD points directly to a commit.
+    if let Ok(git_symbolic_ref_output) = Command::new("git").args(["symbolic-ref", "HEAD"]).output()
+    {
+        if let Ok(symbolic_ref) = String::from_utf8(git_symbolic_ref_output.stdout) {
+            println!("cargo:rerun-if-changed=.git/{symbolic_ref}");
+        } else {
+            println!(
+                "cargo:warning=Could not convert git symbolic-ref output to a UTF-8 string, LIBLOOT_REVISION may become stale!"
+            );
+        }
+    }
+}