home-environment: add hm-version file

This commits adds a file `hm-version` to the generated generation
directory. This file will contain the release version, and if
available, the Git commit hash.

modules/home-environment.nix

@@ -676,6 +676,33 @@ in
 
           ${activationCmds}
         '';
+
+        getVersion = pkgs.writeShellScript "get-hm-version" ''
+          set -euo pipefail
+
+          cd "${../.}" || exit 1
+
+          # Get the base release and initialize an empty version suffix.
+          release=$(< .release)
+          suffix=""
+
+          # If we are in a Git repo then update the suffix to be
+          #
+          #   .git.HASH
+          #
+          # where HASH are the first 8 characters of the commit hash.
+          if [[ -f .git/HEAD ]]; then
+            ref=$(sed '/ref:/ { s/.* //; }' .git/HEAD)
+            if [[ -f ".git/$ref" ]]; then
+              hash=$(< ".git/$ref")
+              if [[ -n "$hash" ]]; then
+                suffix=".git.''${hash:0:8}"
+              fi
+            fi
+          fi
+
+          echo "$release$suffix"
+        '';
       in
         pkgs.runCommand
           "home-manager-generation"
@@ -685,6 +712,8 @@ in
           ''
             mkdir -p $out
 
+            ${getVersion} > $out/hm-version
+
             cp ${activationScript} $out/activate
 
             mkdir $out/bin