Enhance Windows installer to configure itself as helper in all Git installs

[mjcheetham]

The current Windows installer just drops the GCM Core binaries in %ProgramFiles%. We should also look at setting the credential.helper configuration entry, as well as the required useHttpPath = true entry for dev.azure.com remotes.

A few options include:

• Replicate GCM Windows and introduce an install command that discovers all Git installations (including msys installs?) and copies itself into the directory, and sets the credential.helper = manager
• Set the current user's ~/.gitconfig to:

[credential]
    helper = # empty value first to reset the helper list from --system
    helper = <path/to/gcmcore>

• Put %ProgramFiles%\Git Credential Manager on the %PATH%? (problem: how do we support side-by-side GCM Core and GCM Windows if they're both called git-credential-manager.exe?)
• Something else?

These options require answering the question about should GCM Core be a machine-wide or user-scoped install?