Git Sync (Push, Fetch + FF/Merge)

[firu11]

This PR should implement a working Core.SyncAll() method, which will try to synchronize all calendars with its remotes.

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SyncAll does for each calendar:

1. Fetch from remote (if no remote -> skip)
2. Compare "head commits"
3. Merge
   a. Same hashes -> nothing to be done
   b. "Remote head" is ahead -> fast-forward merge aka. append changes to local
   c. Local head is ahead -> push new commits to remote
   d. History diverged -> custom merge strategy -> push merge commit

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Merge conflict resolution strategy

Obvious

Base commit	Local	Remote	Meaning	Result
missing	exists	missing	local added	keep local
missing	missing	exists	remote added	write remote
exists	missing	missing	both deleted	keep deleted

Not so obvious

Base commit	Local	Remote	Meaning	Result
missing	exists	exists	both added	newer UpdatedAt wins
exists	exists modified	exists modified	both edited	newer UpdatedAt wins
exists	missing	exists unchanged	local deleted	keep local delete
exists	exists unchanged	missing	remote deleted	remove local
exists	missing	exists modified	local deleted, remote edited	delete wins
exists	exists modified	missing	local edited, remote deleted	delete wins