hg-core: add Operation interface for high-level hg operations

A distinction is made between operations and commands.
An operation is a high-level function of mercurial whereas a command is what is exposed by the cli.
A single command can use several operations to achieve its goal.

Differential Revision: https://phab.mercurial-scm.org/D8608

rust/hg-core/src/operations/mod.rs

@@ -0,0 +1,9 b''
+/// An interface for high-level hg operations.
+///
+/// A distinction is made between operation and commands.
+/// An operation is what can be done whereas a command is what is exposed by
+/// the cli. A single command can use several operations to achieve its goal.
+pub trait Operation<T> {
+    type Error;
+    fn run(&self) -> Result<T, Self::Error>;
+}

rust/hg-core/src/lib.rs

@@ -23,6 +23,7 b' mod filepatterns;'
 pub mod matchers;
 pub mod revlog;
 pub use revlog::*;
+pub mod operations;
 pub mod utils;
 
 // Remove this to see (potential) non-artificial compile failures. MacOS