feat: Add API from git2/sys/config.h

Added struct `git_config_backend_entry`.
See git2/sys/config.h line 27.

Added struct `git_config_iterator`.
Previously incorrectly defined as an empty enum.
See git2/sys/config.h line 49.

Added struct `git_config_backend`.
See git2/sys/config.h line 69.

Added constant `GIT_CONFIG_BACKEND_VERSION`.
See git2/sys/config.h line 103.

Added struct `git_config_backend_memory_options`.
See git2/sys/config.h line 148.

Added constant `GIT_CONFIG_BACKEND_MEMORY_OPTIONS_VERSION`.
See git2/sys/config.h line 165.

Added function `git_config_add_backend`.
See git2/sys/config.h line 140.

Added function `git_config_backend_from_string`.
See git2/sys/config.h line 181.

Added function `git_config_backend_from_values`.
See git2/sys/config.h line 197.

Added function `git_config_init_backend`.
See git2/sys/config.h line 116.

Author: tecc

libgit2-sys/lib.rs

@@ -55,7 +55,68 @@ pub enum git_branch_iterator {}
 pub enum git_blame {}
 pub enum git_commit {}
 pub enum git_config {}
-pub enum git_config_iterator {}
+
+#[repr(C)]
+pub struct git_config_backend_entry {
+    pub entry: git_config_entry,
+    pub free: Option<extern "C" fn(*mut git_config_backend_entry)>,
+}
+
+#[repr(C)]
+pub struct git_config_iterator {
+    pub backend: *mut git_config_backend,
+    pub flags: c_uint,
+    pub next: Option<
+        extern "C" fn(*mut *mut git_config_backend_entry, *mut git_config_iterator) -> c_int,
+    >,
+    pub free: Option<extern "C" fn(*mut git_config_iterator)>,
+}
+
+#[repr(C)]
+pub struct git_config_backend {
+    pub version: c_uint,
+    pub readonly: c_int,
+    pub cfg: *mut git_config,
+    pub open: Option<
+        extern "C" fn(*mut git_config_backend, git_config_level_t, *const git_repository) -> c_int,
+    >,
+    pub get: Option<
+        extern "C" fn(
+            *mut git_config_backend,
+            *const c_char,
+            *mut *mut git_config_backend_entry,
+        ) -> c_int,
+    >,
+    pub set: Option<extern "C" fn(*mut git_config_backend, *const c_char, *const c_char) -> c_int>,
+    pub set_multivar: Option<
+        extern "C" fn(
+            *mut git_config_backend,
+            *const c_char,
+            *const c_char,
+            *const c_char,
+        ) -> c_int,
+    >,
+    pub del: Option<extern "C" fn(*mut git_config_backend, *const c_char) -> c_int>,
+    pub del_multivar:
+        Option<extern "C" fn(*mut git_config_backend, *const c_char, *const c_char) -> c_int>,
+    pub snapshot:
+        Option<extern "C" fn(*mut *mut git_config_backend, *mut git_config_backend) -> c_int>,
+    pub lock: Option<extern "C" fn(*mut git_config_backend) -> c_int>,
+    pub unlock: Option<extern "C" fn(*mut git_config_backend) -> c_int>,
+    pub free: Option<extern "C" fn(*mut git_config_backend)>,
+}
+
+pub const GIT_CONFIG_BACKEND_VERSION: c_uint = 1;
+
+#[repr(C)]
+pub struct git_config_backend_memory_options {
+    pub version: c_uint,
+    pub backend_type: *const c_char,
+    pub origin_path: *const c_char
+}
+
+pub const GIT_CONFIG_BACKEND_MEMORY_OPTIONS_VERSION: c_uint = 1;
+
 pub enum git_index {}
 pub enum git_index_conflict_iterator {}
 pub enum git_object {}
@@ -98,7 +159,7 @@ pub struct git_odb_stream {
     pub read: Option<extern "C" fn(*mut git_odb_stream, *mut c_char, size_t) -> c_int>,
     pub write: Option<extern "C" fn(*mut git_odb_stream, *const c_char, size_t) -> c_int>,
     pub finalize_write: Option<extern "C" fn(*mut git_odb_stream, *const git_oid) -> c_int>,
-    pub free: Option<extern "C" fn(*mut git_odb_stream)>
+    pub free: Option<extern "C" fn(*mut git_odb_stream)>,
 }
 
 git_enum! {
@@ -3161,13 +3222,32 @@ extern "C" {
     ) -> c_int;
 
     // config
+    pub fn git_config_add_backend(
+        cfg: *mut git_config,
+        file: *mut git_config_backend,
+        level: git_config_level_t,
+        repo: *const git_repository,
+        force: c_int,
+    ) -> c_int;
     pub fn git_config_add_file_ondisk(
         cfg: *mut git_config,
         path: *const c_char,
         level: git_config_level_t,
         repo: *const git_repository,
         force: c_int,
     ) -> c_int;
+    pub fn git_config_backend_backend_from_string(
+        out: *mut *mut git_config_backend,
+        cfg: *const c_char,
+        len: size_t,
+        opts: *mut git_config_backend_memory_options,
+    ) -> c_int;
+    pub fn git_config_backend_backend_from_values(
+        out: *mut *mut git_config_backend,
+        values: *mut *const c_char,
+        len: size_t,
+        opts: *mut git_config_backend_memory_options,
+    ) -> c_int;
     pub fn git_config_delete_entry(cfg: *mut git_config, name: *const c_char) -> c_int;
     pub fn git_config_delete_multivar(
         cfg: *mut git_config,
@@ -3214,6 +3294,7 @@ extern "C" {
         cfg: *const git_config,
         name: *const c_char,
     ) -> c_int;
+    pub fn git_config_init_backend(backend: *mut git_config_backend, version: c_uint) -> c_int;
     pub fn git_config_iterator_free(iter: *mut git_config_iterator);
     pub fn git_config_iterator_glob_new(
         out: *mut *mut git_config_iterator,