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 we're in a git repo and 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 +455,9 @@ 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 -- "$hardwareConfigPath" >/dev/null 2>&1 || true
-    popd
+    if git rev-parse --is-inside-work-tree >/dev/null; then
+      git add --intent-to-add --force -- "$hardwareConfigPath"
+    fi
   fi
 }