rhg: add a limited `rhg root` subcommand

Clap has been choosen for argument parsing for the following reasons:

- it's a wildly used and maintained crate
- it can deal with OS encoding making it suitable for any encoding
- it supports nonambiguous prefix matching as already available in hg
- it will soon allow for structopts-style declarative pattern instead of the
  currently used builder pattern

Differential Revision: https://phab.mercurial-scm.org/D8613

tests/test-rhg.t

@@ -0,0 +1,26 b''
+#require rust
+
+  $ rhg() {
+  > if [ -f "$RUNTESTDIR/../rust/target/debug/rhg" ]; then
+  >   "$RUNTESTDIR/../rust/target/debug/rhg" "$@"
+  > else
+  >   echo "skipped: Cannot find rhg. Try to run cargo build in rust/rhg."
+  >   exit 80
+  > fi
+  > }
+  $ rhg unimplemented-command
+  [252]
+  $ rhg root
+  abort: no repository found in '$TESTTMP' (.hg not found)!
+  [255]
+  $ hg init repository
+  $ cd repository
+  $ rhg root
+  $TESTTMP/repository
+  $ rhg root > /dev/full
+  abort: No space left on device (os error 28)
+  [255]
+  $ rm -rf `pwd`
+  $ rhg root
+  abort: error getting current working directory: $ENOENT$
+  [255]

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rust/rhg/Cargo.toml

@@ -1,9 +1,10 b''
 [package]
 name = "rhg"
 version = "0.1.0"
 authors = ["Antoine Cezar <[e-mail]>"]
 edition = "2018"
 
 [dependencies]
 hg-core = { path = "../hg-core"}
+clap = "2.33.1"
 

rust/rhg/src/main.rs

@@ -1,8 +1,42 b''
+use clap::App;
+use clap::AppSettings;
+use clap::SubCommand;
+
 mod commands;
 mod error;
 mod exitcode;
 mod ui;
+use commands::Command;
 
 fn main() {
+    let mut app = App::new("rhg")
+        .setting(AppSettings::AllowInvalidUtf8)
+        .setting(AppSettings::SubcommandRequired)
+        .setting(AppSettings::VersionlessSubcommands)
+        .version("0.0.1")
+        .subcommand(
+            SubCommand::with_name("root").about(commands::root::HELP_TEXT),
+        );
+
+    let matches = app.clone().get_matches_safe().unwrap_or_else(|_| {
     std::process::exit(exitcode::UNIMPLEMENTED_COMMAND)
+    });
+
+    let command_result = match matches.subcommand_name() {
+        Some(name) => match name {
+            "root" => commands::root::RootCommand::new().run(),
+            _ => std::process::exit(exitcode::UNIMPLEMENTED_COMMAND),
+        },
+        _ => {
+            match app.print_help() {
+                Ok(_) => std::process::exit(exitcode::OK),
+                Err(_) => std::process::exit(exitcode::ABORT),
+            };
 }
+    };
+
+    match command_result {
+        Ok(_) => std::process::exit(exitcode::OK),
+        Err(e) => e.exit(),
+    }
+}