Treat no-op git commit (nothing to commit) as a skip, not a failure - #26

Problem

The action failed on repos where bin/importmap pin produced no changes,
with all planned PRs reporting a hard failure:

• open failed importmap-updates/major-maplibre-gl PR# — git commit ..., status: exit 1, stderr: ""

Root cause

When bin/importmap pin <pkg>@<version> results in no diff (the package
is already at that version), git commit exits with code 1 and writes:

# On branch main
nothing to commit, working tree clean

That message is emitted on stdout, not stderr. But
GitClient#commit_changes only inspected e.result.stderr to detect
this benign case:

rescue Git::FailedError => e
  return false if e.result.stderr.to_s.include?("nothing to commit")
  raise
end

Since stderr was empty, the check never matched, the error was
re-raised, and the action exited 1 — even though there was nothing
wrong, just nothing to commit.

Fix

lib/git_client.rb now checks both stdout and stderr for the
"nothing to commit" marker, so a no-op pin is correctly treated as a
skip ("No changes after pinning") instead of a failure.

rescue Git::FailedError => e
  return false if "#{e.result.stdout} #{e.result.stderr}".include?("nothing to commit")
  raise
end

Test plan

• Added regression test test_commit_changes_returns_false_when_nothing_to_commit_on_stdout
  covering the real git behavior (message on stdout, empty stderr).