build.rs: remove code duplication in version computation

get_version() in build.rs duplicates some logic in
build_tools/git_version_gen.sh. There are some differences

1. When computing the Git hash, get_version() falls back to reading
  .git/HEAD directly if "git describe" fails
1.1. Git is not installed
     Not sure if this is a good strong reason. If you don't have
     Git installed, how would you have created ".git"? If the exact
     Git SHA is important, maybe we should use something like gitoxide
     for this rather than implementing our own.
1.2. there is a permission problem
     The case mentiond in 0083192fcb (Read git SHA ourselves if it
     is unavailable, 2024-12-09) doesn't seem to happen with current
     versions of Git: "sudo git describe" works fine.  Something like
     "sudo -u postgres git describe" doesn't. We could support that
     but let's wait until we know of a use case.
1.3 there are no tags
    (when doing "cargo install --git", as mentioned in 0dfc490721
    (build.rs: Use Cargo_PKG_VERSION if no version could be found,
    2024-06-10)).
    Missing tags are no longer a problem because we read the version
    from Cargo.toml now. Tweak the script to make sure that the
    version is 4.3.3-g${git_sha} instead of just ${git_sha}.

2. get_version() falls back to jj too.
   That was added for jj workspaces that don't have a Git worktree;
   but those should be one their way out; when using jj's Git backend,
   all workspaces will get an associated worktre.

build.rs

@@ -3,7 +3,6 @@
     workspace_root,
 };
 use rsconf::Target;
-use std::env;
 use std::path::{Path, PathBuf};
 
 fn canonicalize<P: AsRef<Path>>(path: P) -> PathBuf {
@@ -35,10 +34,9 @@ fn main() {
     rsconf::set_env_value("BUILD_HOST_TRIPLE", &env_var("HOST").unwrap());
     rsconf::set_env_value("BUILD_PROFILE", &env_var("PROFILE").unwrap());
 
-    let version = &get_version();
     // Per https://doc.rust-lang.org/cargo/reference/build-scripts.html#inputs-to-the-build-script,
     // the source directory is the current working directory of the build script
-    rsconf::set_env_value("FISH_BUILD_VERSION", version);
+    rsconf::set_env_value("FISH_BUILD_VERSION", &get_version());
 
     fish_build_helper::rebuild_if_embedded_path_changed("share");
 
@@ -196,73 +194,14 @@ fn join_if_relative(parent_if_relative: &Path, path: String) -> PathBuf {
 }
 
 fn get_version() -> String {
-    use std::fs::read_to_string;
     use std::process::Command;
-
-    if let Some(var) = env_var("FISH_BUILD_VERSION") {
-        return var;
-    }
-
-    let args = &["describe", "--always", "--dirty=-dirty"];
-    if let Ok(output) = Command::new("git").args(args).output() {
-        let rev = String::from_utf8_lossy(&output.stdout).trim().to_string();
-        if !rev.is_empty() {
-            // If it contains a ".", we have a proper version like "3.7",
-            // or "23.2.1-1234-gfab1234"
-            if rev.contains('.') {
-                return rev;
-            }
-            // If it doesn't, we probably got *just* the commit SHA,
-            // like "f1242abcdef".
-            // So we prepend the crate version so it at least looks like
-            // "3.8-gf1242abcdef"
-            // This lacks the commit *distance*, but that can't be helped without
-            // tags.
-            let version = env!("CARGO_PKG_VERSION").to_owned();
-            return version + "-g" + &rev;
-        }
-    }
-
-    // git did not tell us a SHA either because it isn't installed,
-    // or because it refused (safe.directory applies to `git describe`!)
-    // So we read the SHA ourselves.
-    fn get_git_hash() -> Result<String, Box<dyn std::error::Error>> {
-        let workspace_root = workspace_root();
-        let gitdir = workspace_root.join(".git");
-        let jjdir = workspace_root.join(".jj");
-        let commit_id = if gitdir.exists() {
-            // .git/HEAD contains ref: refs/heads/branch
-            let headpath = gitdir.join("HEAD");
-            let headstr = read_to_string(headpath)?;
-            let headref = headstr.split(' ').nth(1).unwrap().trim();
-
-            // .git/refs/heads/branch contains the SHA
-            let refpath = gitdir.join(headref);
-            // Shorten to 9 characters (what git describe does currently)
-            read_to_string(refpath)?
-        } else if jjdir.exists() {
-            let output = Command::new("jj")
-                .args([
-                    "log",
-                    "--revisions",
-                    "@",
-                    "--no-graph",
-                    "--ignore-working-copy",
-                    "--template",
-                    "commit_id",
-                ])
-                .output()
-                .unwrap();
-            String::from_utf8_lossy(&output.stdout).to_string()
-        } else {
-            return Err("did not find either of .git or .jj".into());
-        };
-        let refstr = &commit_id[0..9];
-        let refstr = refstr.trim();
-
-        let version = env!("CARGO_PKG_VERSION").to_owned();
-        Ok(version + "-g" + refstr)
-    }
-
-    get_git_hash().expect("Could not get a version. Either set $FISH_BUILD_VERSION or install git.")
+    String::from_utf8(
+        Command::new("build_tools/git_version_gen.sh")
+            .output()
+            .unwrap()
+            .stdout,
+    )
+    .unwrap()
+    .trim_ascii_end()
+    .to_string()
 }