FR: Narrowly-scoped, TTL and/or password based access to the store

[SpiderUnderUrBed]

Related to #2359
As i explained in that discussion, I would like to store some of my credentials on gcm, but i would like it so i could better run this in a not-entirely trusted environment, not untrusted either, example being, a VPS where me and other people are working on various projects, whether it be narrowly scoped, TTL based or more ideal for my use case, a password required to unlock a store or decrypt credentials kind of like how SSH does it, would be useful for my use case and im sure other people could benefit also.

Some of these improvements were mentioned by krotname
"The current documented choices already include credential.credentialStore none and Git's cache store with credential.cacheOptions --timeout ..., but there is not a first-class GCM mode that clearly means: keep credentials only for this login/session, behave predictably over SSH/shared hosts, and clean up on logout or after a configured TTL.

That would also make it easier to document the recommended setup for shared servers without sending people through several Git/GCM settings."