Generally, no, mentioned git signatures do not make you more vulnerable to impersonation during SSH authentication. For SSH authentication, the attacker would have to know the private key. And the security of the private key does not depend on how many different messages you sign with it, since it cannot be derived from signatures. That is, as long as the key algorithm is secure, such as
ed25519, or RSA with at least 2048 bits.
I also can't see a way how an attacker could correlate the signed git-hashes and SSH authentication in such a way that by signing the one you are signing the other. I don't know if the key exchange mechanism would even allow for this, but even if it did, git hashes contain a random / time based component, and so does the key exchange with an SSH server. So for an attacker to be able to fabricate git hashes on your local system that correspond with real-time SSH authentication requests, and having you sign the same, he would need such a high level of control over your system that there would be more direct ways to access your private keys. If for example, he intercepts the
git process, or places a modified version of the executable on your system, then he might as well intercept the key itself.
No, using the same key for
ssh does not make you vulnerable to impersonation.