credential: change git_cred to git_credential

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.

include/git2/credential_helpers.h

@@ -0,0 +1,52 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_git_credential_helpers_h__
+#define INCLUDE_git_credential_helpers_h__
+
+#include "transport.h"
+
+/**
+ * @file git2/credential_helpers.h
+ * @brief Utility functions for credential management
+ * @defgroup git_credential_helpers credential management helpers
+ * @ingroup Git
+ * @{
+ */
+GIT_BEGIN_DECL
+
+/**
+ * Payload for git_credential_userpass_plaintext.
+ */
+typedef struct git_credential_userpass_payload {
+	const char *username;
+	const char *password;
+} git_credential_userpass_payload;
+
+
+/**
+ * Stock callback usable as a git_credential_acquire_cb.  This calls
+ * git_cred_userpass_plaintext_new unless the protocol has not specified
+ * `GIT_CREDENTIAL_USERPASS_PLAINTEXT` as an allowed type.
+ *
+ * @param out The newly created credential object.
+ * @param url The resource for which we are demanding a credential.
+ * @param user_from_url The username that was embedded in a "user\@host"
+ *                          remote url, or NULL if not included.
+ * @param allowed_types A bitmask stating which credential types are OK to return.
+ * @param payload The payload provided when specifying this callback.  (This is
+ *        interpreted as a `git_credential_userpass_payload*`.)
+ */
+GIT_EXTERN(int) git_credential_userpass(
+		git_credential **out,
+		const char *url,
+		const char *user_from_url,
+		unsigned int allowed_types,
+		void *payload);
+
+/** @} */
+GIT_END_DECL
+#endif