nix: send the Git date to build.rs

Thanks to @thoughtpolice for providing the original version of the nix code; I
edited it a bit.

cli/build.rs

@@ -56,11 +56,13 @@ fn main() -> std::io::Result<()> {
 
 /// Return the committer date in YYYYMMDD format and the git hash
 fn get_git_date_and_hash() -> Option<(String, String)> {
-    if let Some(nix_hash) = std::env::var("NIX_JJ_GIT_HASH")
-        .ok()
-        .filter(|s| !s.is_empty())
-    {
-        return Some(("nix".to_string(), nix_hash));
+    if let (Ok(nix_hash), Ok(nix_date)) = (
+        std::env::var("NIX_JJ_GIT_HASH"),
+        std::env::var("NIX_JJ_GIT_DATE"),
+    ) {
+        if !nix_hash.is_empty() && !nix_date.is_empty() {
+            return Some((nix_date, nix_hash));
+        }
     }
 
     fn trim_and_split_on_vbar(bytes: &[u8]) -> (String, String) {

cli/tests/test_global_opts.rs

@@ -71,7 +71,7 @@ fn test_no_subcommand() {
     let sanitized = stdout.replace(|c: char| c.is_ascii_hexdigit(), "?");
     assert_matches!(
         sanitized.as_str(),
-        "jj ?.?.?\n" | "jj ?.?.?-????????-????????????????\n" | "jj ?.?.?-nix-????????????????\n"
+        "jj ?.?.?\n" | "jj ?.?.?-????????-????????????????\n"
     );
 
     let stdout = test_env.jj_cmd_success(test_env.env_root(), &["-R", "repo"]);

flake.nix

@@ -18,6 +18,11 @@
   } //
   (flake-utils.lib.eachDefaultSystem (system:
     let
+      # TODO(aseipp): Use dirtyRev and dirtyShortRev to record dirty checkout
+      # when we update `build.rs` to understand dirty checkouts.
+      gitRev = self.rev or "";
+      gitDate = builtins.substring 0 8 (self.lastModifiedDate or self.lastModified or "");
+
       pkgs = import nixpkgs {
         inherit system;
         overlays = [
@@ -72,10 +77,11 @@
             darwin.apple_sdk.frameworks.Security
             darwin.apple_sdk.frameworks.SystemConfiguration
             libiconv
-            ];
+          ];
           ZSTD_SYS_USE_PKG_CONFIG = "1";
           LIBSSH2_SYS_USE_PKG_CONFIG = "1";
-          NIX_JJ_GIT_HASH = self.rev or "";
+          NIX_JJ_GIT_HASH = gitRev;
+          NIX_JJ_GIT_DATE = gitDate;
           CARGO_INCREMENTAL = "0";
           postInstall = ''
             $out/bin/jj util mangen > ./jj.1
@@ -136,6 +142,8 @@
           export RUST_BACKTRACE=1
           export ZSTD_SYS_USE_PKG_CONFIG=1
           export LIBSSH2_SYS_USE_PKG_CONFIG=1
+          export NIX_JJ_GIT_HASH="${gitRev}"
+          export NIX_JJ_GIT_DATE="${gitDate};
         '';
       };
     }));