lg2: use cfgwr_commit to write to configs using libgit2

lg2: use cfgwr_commit to write to configs using libgit2

libgit2 allows us to avoid process spawning overhead when
writing to the config file.  While the performance difference is
unlikely to matter in real-world use, it should improve
maintainability of tests by allowing us to write tests with
`public-inbox-init' with a smaller performance penalty (as
opposed to using `PublicInbox::IO::write_file' to setup
configs).