Add initial command line interface

Users can now use the --against argument to specify against which
commit/tag/branch the current api must be compared against.

Cargo.toml

@@ -23,3 +23,4 @@ anyhow = "1.0"
 git2 = "0.13"
 cargo_toml = "0.9"
 semver = "1.0"
+clap = "2.33"

src/cli.rs

@@ -0,0 +1,26 @@
+use clap::{crate_authors, crate_description, crate_name, crate_version, App, Arg};
+
+pub(crate) struct ProgramConfig {
+    pub comparaison_ref: String,
+}
+
+impl ProgramConfig {
+    pub(crate) fn parse() -> ProgramConfig {
+        let matches = App::new(crate_name!())
+            .version(crate_version!())
+            .author(crate_authors!())
+            .about(crate_description!())
+            .arg(
+                Arg::with_name("against")
+                    .short("a")
+                    .help("Sets the git reference to compare the API against. Can be a tag, a branch name or a commit.")
+                    .takes_value(true)
+                    .required(false)
+                    .default_value("main")
+            ).get_matches();
+
+        let comparaison_ref = matches.value_of("against").unwrap().to_owned();
+
+        ProgramConfig { comparaison_ref }
+    }
+}

src/git.rs

@@ -2,8 +2,6 @@ use anyhow::{Context, Result as AnyResult};
 
 use git2::{Repository, StashFlags, StatusOptions};
 
-pub(crate) static DEFAULT_BRANCH_NAME: &str = "main";
-
 pub(crate) trait GitBackend: Sized {
     fn switch_to(&mut self, id: &str) -> AnyResult<()> {
         if self.needs_stash() {

src/lib.rs

@@ -1,4 +1,5 @@
 mod ast;
+mod cli;
 mod comparator;
 mod git;
 mod glue;
@@ -14,13 +15,15 @@ use crate::{
 };
 
 pub fn run() -> AnyResult<()> {
+    let config = cli::ProgramConfig::parse();
+
     let mut repo = CrateRepo::current().context("Failed to fetch repository data")?;
 
     let current_api = glue::extract_api().context("Failed to get crate API")?;
     let version = manifest::get_crate_version().context("Failed to get crate version")?;
 
-    repo.switch_to(git::DEFAULT_BRANCH_NAME)
-        .with_context(|| format!("Failed to checkout to `{}`", git::DEFAULT_BRANCH_NAME))?;
+    repo.switch_to(config.comparaison_ref.as_str())
+        .with_context(|| format!("Failed to checkout to `{}`", config.comparaison_ref))?;
 
     let previous_api = glue::extract_api().context("Failed to get crate API")?;