Extracts the git env logic in snapshot.go (#760)

Author: mcncl

internal/preflight/git.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
+	"slices"
 	"strings"
 )
 
@@ -46,3 +47,13 @@ func gitOutput(dir string, env []string, debug bool, args ...string) (string, er
 func RepositoryRoot(dir string, debug bool) (string, error) {
 	return gitOutput(dir, nil, debug, "rev-parse", "--show-toplevel")
 }
+
+// tempIndexEnv returns a copy of the current environment with GIT_INDEX_FILE
+// set to path, stripping any existing GIT_INDEX_FILE entry. This is used to
+// direct git commands at a temporary index without affecting the real one.
+func tempIndexEnv(path string) []string {
+	env := slices.DeleteFunc(os.Environ(), func(e string) bool {
+		return strings.HasPrefix(e, "GIT_INDEX_FILE=")
+	})
+	return append(env, "GIT_INDEX_FILE="+path)
+}

internal/preflight/snapshot.go

@@ -69,13 +69,7 @@ func Snapshot(dir string, preflightID uuid.UUID, opts ...SnapshotOption) (*Snaps
 	tmp.Close()
 	defer os.Remove(tmpIndex)
 
-	var env []string
-	for _, e := range os.Environ() {
-		if !strings.HasPrefix(e, "GIT_INDEX_FILE=") {
-			env = append(env, e)
-		}
-	}
-	env = append(env, fmt.Sprintf("GIT_INDEX_FILE=%s", tmpIndex))
+	env := tempIndexEnv(tmpIndex)
 
 	// Seed the temp index from HEAD.
 	if err := gitRun(dir, env, cfg.debug, "read-tree", "HEAD"); err != nil {