Ensure git is executed inside the gemspec dir
Ensure git is executed inside the gemspec dir

When a Gemfile was pointing to a local gem using git to list its files the git command
was executed from within the wrong dir. This fixes the issue without:

1. introducing new dependencies (e.g. `shellwords`)
2. any risks with non-trivial paths (e.g. paths containing spaces)
mvz/bundler,bundler/bundler,bundler/bundler,mvz/bundler,dilumnavanjana/bundler,dilumnavanjana/bundler