rust-core: extracted a revlog submodule

rust-core: extracted a revlog submodule

This moves fundamental definitions from the top of the crate
to the newly created `revlog` submodule and reexports them for
easy compatibility.

As we are about to add new features to this crate, we felt it
will improve clarity, and moreso if `ancestors` and `dagops`
would become submodules of `revlog`.

Differential Revision: https://phab.mercurial-scm.org/D7782

--- a/rust/hg-core/src/lib.rs	Thu Dec 19 00:32:42 2019 -0800
+++ b/rust/hg-core/src/lib.rs	Wed Dec 25 15:17:55 2019 +0100
@@ -19,6 +19,8 @@
 };
 mod filepatterns;
 pub mod matchers;
+pub mod revlog;
+pub use revlog::*;
 pub mod utils;
 
 use crate::utils::hg_path::HgPathBuf;
@@ -28,32 +30,6 @@
 use std::collections::HashMap;
 use twox_hash::RandomXxHashBuilder64;
 
-/// Mercurial revision numbers
-///
-/// As noted in revlog.c, revision numbers are actually encoded in
-/// 4 bytes, and are liberally converted to ints, whence the i32
-pub type Revision = i32;
-
-/// Marker expressing the absence of a parent
-///
-/// Independently of the actual representation, `NULL_REVISION` is guaranteed
-/// to be smaller that all existing revisions.
-pub const NULL_REVISION: Revision = -1;
-
-/// Same as `mercurial.node.wdirrev`
-///
-/// This is also equal to `i32::max_value()`, but it's better to spell
-/// it out explicitely, same as in `mercurial.node`
-pub const WORKING_DIRECTORY_REVISION: Revision = 0x7fffffff;
-
-/// The simplest expression of what we need of Mercurial DAGs.
-pub trait Graph {
-    /// Return the two parents of the given `Revision`.
-    ///
-    /// Each of the parents can be independently `NULL_REVISION`
-    fn parents(&self, rev: Revision) -> Result<[Revision; 2], GraphError>;
-}
-
 pub type LineNumber = usize;
 
 /// Rust's default hasher is too slow because it tries to prevent collision
@@ -62,12 +38,6 @@
 pub type FastHashMap<K, V> = HashMap<K, V, RandomXxHashBuilder64>;
 
 #[derive(Clone, Debug, PartialEq)]
-pub enum GraphError {
-    ParentOutOfRange(Revision),
-    WorkingDirectoryUnsupported,
-}
-
-#[derive(Clone, Debug, PartialEq)]
 pub enum DirstateParseError {
     TooLittleData,
     Overflow,

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hg-core/src/revlog.rs	Wed Dec 25 15:17:55 2019 +0100
@@ -0,0 +1,38 @@
+// Copyright 2018-2020 Georges Racinet <[e-mail]>
+//           and Mercurial contributors
+//
+// This software may be used and distributed according to the terms of the
+// GNU General Public License version 2 or any later version.
+//! Mercurial concepts for handling revision history
+
+/// Mercurial revision numbers
+///
+/// As noted in revlog.c, revision numbers are actually encoded in
+/// 4 bytes, and are liberally converted to ints, whence the i32
+pub type Revision = i32;
+
+/// Marker expressing the absence of a parent
+///
+/// Independently of the actual representation, `NULL_REVISION` is guaranteed
+/// to be smaller that all existing revisions.
+pub const NULL_REVISION: Revision = -1;
+
+/// Same as `mercurial.node.wdirrev`
+///
+/// This is also equal to `i32::max_value()`, but it's better to spell
+/// it out explicitely, same as in `mercurial.node`
+pub const WORKING_DIRECTORY_REVISION: Revision = 0x7fffffff;
+
+/// The simplest expression of what we need of Mercurial DAGs.
+pub trait Graph {
+    /// Return the two parents of the given `Revision`.
+    ///
+    /// Each of the parents can be independently `NULL_REVISION`
+    fn parents(&self, rev: Revision) -> Result<[Revision; 2], GraphError>;
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum GraphError {
+    ParentOutOfRange(Revision),
+    WorkingDirectoryUnsupported,
+}