Stock callback usable as a git_cred_acquire_cb. This calls
git_cred_userpass_plaintext_new unless the protocol has not specified
GIT_CREDTYPE_USERPASS_PLAINTEXT
as an allowed type.
The resource for which we are demanding a credential.
The username that was embedded in a "user@host" remote url, or NULL if not included.
A bitmask stating which cred types are OK to return.
The payload provided when specifying this callback. (This is interpreted as a git_cred_userpass_payload*
.)