[flakes] add flakes.nix to git only if we are in a git repo (#539)

## Summary

This PR looks if the devbox project is in a git repository before adding
the
flake.nix to a "fake git repository". With this PR, `devbox` with flakes
will no longer require `git` as a hard dependency.

When I tested with a simple flakes playground project, I observed:
1. if not in a git repository, then flakes.nix can be used by `nix
develop`.
2. if in a git repository, then `nix develop` fails if flakes.nix is not
tracked.
3. Surprisingly, if in a mercurial repository, then `nix develop` still
works if flakes.nix is not tracked by mercurial. Surprising because docs
online hint that flakes supports both git and mercurial, but I guess not
enforced for this requirement.

## How was it tested?

Ran `DEVBOX_FEATURE_FLAKES=1 devbox shell` in devbox projects:
1. in a git repo 
2. outside a git repo

Author: savil

internal/impl/generate.go

@@ -36,8 +36,6 @@ func generateForShell(rootPath string, plan *plansdk.ShellPlan, pluginManager *p
 	}
 
 	// Gitignore file is added to the .devbox directory
-	// TODO savil. Remove this hardcode from here, so this function can be generically defined again
-	//    by accepting the files list parameter.
 	err := writeFromTemplate(filepath.Join(rootPath, ".devbox"), plan, ".gitignore")
 	if err != nil {
 		return errors.WithStack(err)
@@ -112,6 +110,12 @@ func makeFlakeFile(outPath string, plan *plansdk.ShellPlan) error {
 		return errors.WithStack(err)
 	}
 
+	if !isProjectInGitRepo(outPath) {
+		// if we are not in a git repository, then carry on
+		return nil
+	}
+	// if we are in a git repository, then nix requires that the flake.nix file be tracked by git
+
 	// make an empty git repo
 	// Alternatively consider: git add intent-to-add path/to/flake.nix, and
 	// git update-index --assume-unchanged path/to/flake.nix
@@ -136,3 +140,24 @@ func makeFlakeFile(outPath string, plan *plansdk.ShellPlan) error {
 	}
 	return nil
 }
+
+func isProjectInGitRepo(dir string) bool {
+
+	for dir != "/" {
+		// Look for a .git directory in `dir`
+		if _, err := os.Stat(filepath.Join(dir, ".git")); err == nil {
+			// Found a .git
+			return true
+		} else if !os.IsNotExist(err) {
+			// An error means we will not find a git repo so return false
+			return false
+		} else {
+			// No .git directory found, so loop again into the parent dir
+			dir = filepath.Dir(dir)
+			continue
+		}
+	}
+	// We reached the fs-root dir, climbed the highest mountain and
+	// we still haven't found what we're looking for.
+	return false
+}