We avoid abbreviations where possible; rename git_cred to
git_credential.
In addition, we have standardized on a trailing `_t` for enum types,
instead of using "type" in the name. So `git_credtype_t` has become
`git_credential_t` and its members have become `GIT_CREDENTIAL` instead
of `GIT_CREDTYPE`.
Finally, the source and header files have been renamed to `credential`
instead of `cred`.
Keep previous name and values as deprecated, and include the new header
files from the previous ones.
