Introduce git_libgit2_feature_backend API

Provide a mechanism to understand the backend provider for feature
within libgit2. For example, one can query the mechanism that provides
HTTPS by asking for the backend for the `GIT_FEATURE_HTTPS`.

This is particularly useful for features that are not completely
isomorphic; the HTTPS providers may have slightly different
functionality that can be controlled (eg, certificates or cipher
support). And the SSH feature is _very_ different between libssh2 and
OpenSSH.

It may also be useful to understand the support for things like the SHA1
or SHA256 backends to ensure that sha1dc is used, or that FIPS mode is
enabled.

include/git2/common.h

@@ -130,57 +130,81 @@ GIT_EXTERN(int) git_libgit2_version(int *major, int *minor, int *rev);
 GIT_EXTERN(const char *) git_libgit2_prerelease(void);
 
 /**
- * Combinations of these values describe the features with which libgit2
- * was compiled
+ * Configurable features of libgit2; either optional settings (like
+ * threading), or features that can be enabled by one of a number of
+ * different backend "providers" (like HTTPS, which can be provided by
+ * OpenSSL, mbedTLS, or system libraries).
  */
 typedef enum {
-  /**
-   * If set, libgit2 was built thread-aware and can be safely used from multiple
-   * threads.
-   */
-	GIT_FEATURE_THREADS	= (1 << 0),
-  /**
-   * If set, libgit2 was built with and linked against a TLS implementation.
-   * Custom TLS streams may still be added by the user to support HTTPS
-   * regardless of this.
-   */
-	GIT_FEATURE_HTTPS	= (1 << 1),
-  /**
-   * If set, libgit2 was built with and linked against libssh2. A custom
-   * transport may still be added by the user to support libssh2 regardless of
-   * this.
-   */
-	GIT_FEATURE_SSH		= (1 << 2),
-  /**
-   * If set, libgit2 was built with support for sub-second resolution in file
-   * modification times.
-   */
-	GIT_FEATURE_NSEC	= (1 << 3)
+	/**
+	 * libgit2 is thread-aware and can be used from multiple threads
+	 * (as described in the documentation).
+	 */
+	GIT_FEATURE_THREADS        = (1 << 0),
+
+	/** HTTPS remotes */
+	GIT_FEATURE_HTTPS          = (1 << 1),
+
+	/** SSH remotes */
+	GIT_FEATURE_SSH	           = (1 << 2),
+
+	/** Sub-second resolution in index timestamps */
+	GIT_FEATURE_NSEC           = (1 << 3),
+
+	/** HTTP parsing; always available */
+	GIT_FEATURE_HTTP_PARSER    = (1 << 4),
+
+	/** Regular expression support; always available */
+	GIT_FEATURE_REGEX          = (1 << 5),
+
+	/** Internationalization support for filename translation */
+	GIT_FEATURE_I18N           = (1 << 6),
+
+	/** NTLM support over HTTPS */
+	GIT_FEATURE_AUTH_NTLM      = (1 << 7),
+
+	/** Kerberos (SPNEGO) authentication support over HTTPS */
+	GIT_FEATURE_AUTH_NEGOTIATE = (1 << 8),
+
+	/** zlib support; always available */
+	GIT_FEATURE_COMPRESSION    = (1 << 9),
+
+	/** SHA1 object support; always available */
+	GIT_FEATURE_SHA1           = (1 << 10),
+
+	/** SHA256 object support */
+	GIT_FEATURE_SHA256         = (1 << 11)
 } git_feature_t;
 
 /**
  * Query compile time options for libgit2.
  *
  * @return A combination of GIT_FEATURE_* values.
- *
- * - GIT_FEATURE_THREADS
- *   Libgit2 was compiled with thread support. Note that thread support is
- *   still to be seen as a 'work in progress' - basic object lookups are
- *   believed to be threadsafe, but other operations may not be.
- *
- * - GIT_FEATURE_HTTPS
- *   Libgit2 supports the https:// protocol. This requires the openssl
- *   library to be found when compiling libgit2.
- *
- * - GIT_FEATURE_SSH
- *   Libgit2 supports the SSH protocol for network operations. This requires
- *   the libssh2 library to be found when compiling libgit2
- *
- * - GIT_FEATURE_NSEC
- *   Libgit2 supports the sub-second resolution in file modification times.
  */
 GIT_EXTERN(int) git_libgit2_features(void);
 
+/**
+ * Query the backend details for the compile-time feature in libgit2.
+ *
+ * This will return the "backend" for the feature, which is useful for
+ * things like HTTPS or SSH support, that can have multiple backends
+ * that could be compiled in.
+ *
+ * For example, when libgit2 is compiled with dynamic OpenSSL support,
+ * the feature backend will be `openssl-dynamic`. The feature backend
+ * names reflect the compilation options specified to the build system
+ * (though in all lower case). The backend _may_ be "builtin" for
+ * features that are provided by libgit2 itself.
+ *
+ * If the feature is not supported by the library, this API returns
+ * `NULL`.
+ *
+ * @param feature the feature to query details for
+ * @return the provider details, or NULL if the feature is not supported
+ */
+GIT_EXTERN(const char *) git_libgit2_feature_backend(
+	git_feature_t feature);
+
 /**
  * Global library options
  *