hardware config: fix git add logic

Fixing the git add hardware config logic in case the user provides a
relative path. We're doing two things:

1. Failing loudly if the git add command fails. At that point, we
   already know git is in $PATH. If the git add fails, the evaluation
   will likely fail in a flake setting. Better fail early with a helpful
   git error message.
2. We're resolving the hardware config path in the CLI parsing layer
   to make sure this path has the meaning the user intented it to have.

src/nixos-anywhere.sh

@@ -176,7 +176,7 @@ parseArgs() {
         abort "Unknown hardware config backend: $2"
         ;;
       esac
-      hardwareConfigPath=$3
+      hardwareConfigPath=$(realpath $3)
       shift
       shift
       ;;
@@ -455,9 +455,7 @@ generateHardwareConfig() {
 
   # to make sure nix knows about the new file
   if command -v git >/dev/null; then
-    pushd "$(dirname "$hardwareConfigPath")"
-    git add --intent-to-add --force -- "$hardwareConfigPath" >/dev/null 2>&1 || true
-    popd
+    git add --intent-to-add --force -- "$hardwareConfigPath"
   fi
 }