Support developers taking over the dev environment.

[jlewi]

• See https://github.com/jlewi/hydros/issues/9

• This PR adds the command hydros takeover ... which can be used to hydrate based on the locally modified changes by taking over the deve environment.

• This PR adds a PushLocal function which will automatically push any changes to the SourceRepo branch listed in a ManifestSync

• The Syncer can then be used to apply the Manifest in order to push the local changes to the dev environment.

• Fix an issue in RepoHelper with .gitignore not being respected when adding files

  • We need to explicitly load and parse the .gitignore files and add the expresions to the worktree
  • go-gits implementation doesn't do this by default.

• Start a directory to contain documentation.