pkg/vcs: export GitWrapper

[a-nogikh]

One more part of #5620 (see the discussion there as well).

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It will let other parts of the code to use only the git-specific functionality.

[tarasmadan]

Commit comment doesn't explain what's going on.
Please make it valuable for the future readers.

[tarasmadan]

Please start from the explanation why this git wrapper is needed.

[a-nogikh]

Updated the commit description to

    pkg/vcs: export GitWrapper
    
    The current common vcs interface and its implementations are designed to
    support a very big number of situations (git modules, cleaning up
    complex state, etc), which is too heavy and restrictive when we just
    need a thin wrapper over the git command that supports a few basic
    operations.
    
    Refactor pkg/vcs to split out the wrapper code from the big git
    implementation of the vcs.Repo interface.
    
    Export the wrapper to enable reuse in other parts of the system.

Re. the usages -- see syz-cluster/pkg/triage/git.go in #5620.

Specifically, I could not use pkg/vcs's generic Patch because it would not support raw emails (while git apply does), so I needed a wrapper over the git command. The wrapper code was already incorporated into pkg/vcs's git, so instead of duplicating the code I split it out from the git implementation.