Switch to go-git for all remote git interactions including auth (issue #651)

[jessesuen]

Switches all interactions with a git remote to use go-git (instead of git CLI) which provides the following benefits:

• resolves issue Credentials not being accepted for Google Source Repositories #651 where Google Source Repository credentials were not being accepted
• we never store credentials on disk in the repo server (or anywhere else for that matter)
• eliminates the extra git process forks we currently perform with respect to managing credentials

Also fixes an issue where argocd-server logged credentials in plain text during repo add (issue #653)

Performed the following tests:

• private credentials to GSR
• git with SSH private key
• public repos
• verified updates to HEAD, branch, and git tags were seen

[merenbach]

Looks good! Just out of curiosity, what does the line where we're setting HOME=/dev/null in the checkout command environment do?

[jessesuen]

Looks good! Just out of curiosity, what does the line where we're setting HOME=/dev/null in the checkout command environment do?

it makes dev environment closer to the actual environment by not picking up gitconfigs in user's home directory.