Gracefully fails whenunable to parse old api

Previous system stopped the whole program at the first error, which
sometimes stopped the program while HEAD was pointing to an old commit.
This commit ensures that we always (try to) checkout back to the initial
git reference before stopping the program execution.

src/git.rs

@@ -3,6 +3,25 @@ use anyhow::{Context, Result as AnyResult};
 use git2::{Repository, StashFlags, StatusOptions};
 
 pub(crate) trait GitBackend: Sized {
+    fn run_in<F, O>(&mut self, id: &str, f: F) -> AnyResult<O>
+    where
+        F: FnOnce() -> O,
+    {
+        self.switch_to(id)
+            .with_context(|| format!("Failed to checkout to {}", id))?;
+
+        let rslt = f();
+
+        self.switch_back().with_context(|| {
+            format!(
+                "Failed to switch back to {}",
+                self.previous_branch().unwrap()
+            )
+        })?;
+
+        Ok(rslt)
+    }
+
     fn switch_to(&mut self, id: &str) -> AnyResult<()> {
         if self.needs_stash() {
             self.stash_push().context("Failed to stash changes")?;

src/lib.rs

@@ -19,16 +19,13 @@ pub fn run() -> AnyResult<()> {
 
     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(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")?;
+    let current_api = glue::extract_api().context("Failed to get crate API")?;
 
-    repo.switch_back()
-        .context("Failed to go back to initial branch")?;
+    let previous_api = repo.run_in(config.comparaison_ref.as_str(), || {
+        glue::extract_api().context("Failed to get crate API")
+    })??;
 
     let api_comparator = ApiComparator::new(previous_api, current_api);