If an error occurs when calling git_revparse_single() with revspec->to, revspec->from must be freed, since it was already successfully allocated.

Fixes #7184.

I didn't set revspec->from to NULL after freeing it, since I don't generally see that pattern throughout the repo.

It's possible revspec->from was not being freed on purpose, since the object isn't opaque, and after receiving an error code, the caller can check if it is non-NULL and free it. I leaned more toward making the function atomic, but I can see how the original behavior may be preferred, especially since the git_revspec docs explicitly mention that freeing from and to is up to the caller.

If the original behavior is preferred, I could instead add a note to the git_revparse docs about this. Something like:

If an error occurs parsing to, make sure to free from.

Just to highlight that an error does not necessarily mean nothing has been allocated.