RepoCommand: Offer to set extra files in the destination repository

RepoCommand: Offer to set extra files in the destination repository

We want to save in the destination repository what manifest created its
structure. This helps to detect and debug failures in the manifest ->
superproject translations. The src commit should be easily readable from
the superproject tip.

Offer an API to write a file in the destination repository. RepoCommand
callers (e.g. gerrit supermanifest plugin) can use this to add a
file with the repo/ref/hash of the manifest.

Alternatives considered to write the source repo/ref/hash:

  * .gitattributes of the .gitmodules file. Some updates in the manifest
    don't touch the .gitmodules (e.g. a linkfile change), so it can fall
    out of sync.

  * commit message. Caller would need to follow the commit history to
    find the latest modification by repo command. This is not helpful
    e.g. for build bots that want to get the value in one call.

Change-Id: I113662734a7ccd39cbc60b46ad3f73038c807682