Consider using one git mirroring implementation

[squaremo]

The helm operator has a similar, essential requirement to the flux daemon, that of mirroring a git repo. Because the git package was very coupled to how the daemon worked, at the time, they use entirely separate packages. Now the git package has been rewritten, it's plausible that it could be adapted for use in both the helm operator and the daemon.

[ncabatoff]

It probably makes more sense to do #957 instead given the other simplifications it allows.

[squaremo]

Fixed in #1240.