Add --skip-git option to init cli command

[travs]

Automating git commands inside a script or cli tool can lead to confusing and unwanted behaviour.

This PR allows the user to skip git automation when initing a new subgraph.
Personally I think the git automation should be opt-in instead of the default, but I left the default behaviour as-is for an easy merge.

Related:

• show graceful errors if users do not have git installed #926
• graph init fails if git credentials aren't globally set #775
• Do not create .git folder after running graph init #1450

[changeset-bot]

🦋 Changeset detected

Latest commit: 1658fcf

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@graphprotocol/graph-cli	Minor

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[saihaj]

Thank you!

I was also thinking today about adding someway to disallow git, maybe in next major we should not init git default and have users pass in a flag