Don't allow Bounds::Commits to contain non-hashes.

There was only ever one case of it containing a non-hash, and that was
when `--start` was a hash, and `--end` was not specified. In that case,
it was "origin/master", and then converted to a hash later on. To
simplify things, this converts `origin/master` to master's actual commit
early when the `Bounds` is created.

Author: ehuss

Diff for: src/bounds.rs

@@ -61,6 +61,8 @@ pub enum Bounds {
     /// date where the regression does not occur.
     SearchNightlyBackwards { end: GitDate },
     /// Search between two commits.
+    ///
+    /// `start` and `end` must be SHA1 hashes of the commit object.
     Commits { start: String, end: String },
     /// Search between two dates.
     Dates { start: GitDate, end: GitDate },
@@ -91,7 +93,7 @@ impl Bounds {
             }
             (Some(Bound::Commit(start)), None) => Bounds::Commits {
                 start,
-                end: "origin/master".to_string(),
+                end: args.access.repo().commit("origin/master")?.sha,
             },
             (None, Some(Bound::Commit(end))) => Bounds::Commits {
                 start: EPOCH_COMMIT.to_string(),

Diff for: src/main.rs

@@ -947,10 +947,10 @@ impl Config {
         self.bisect_ci_via(start, end)
     }
 
-    fn bisect_ci_via(&self, start_sha: &str, end_ref: &str) -> anyhow::Result<BisectionResult> {
+    fn bisect_ci_via(&self, start_sha: &str, end_sha: &str) -> anyhow::Result<BisectionResult> {
         let access = self.args.access.repo();
         let start = access.commit(start_sha)?;
-        let end = access.commit(end_ref)?;
+        let end = access.commit(end_sha)?;
         let assert_by_bors = |c: &Commit| -> anyhow::Result<()> {
             if c.committer.name != BORS_AUTHOR {
                 bail!(
@@ -967,7 +967,15 @@ impl Config {
         assert_by_bors(&end)?;
         let commits = access.commits(start_sha, &end.sha)?;
 
-        assert_eq!(commits.last().expect("at least one commit").sha, end.sha);
+        let Some(last) = commits.last() else {
+            bail!("expected at least one commit");
+        };
+        if !last.sha.starts_with(&end.sha) {
+            bail!(
+                "expected the last commit to be {end_sha}, but got {}",
+                last.sha
+            );
+        }
 
         commits.iter().zip(commits.iter().skip(1)).all(|(a, b)| {
             let sorted_by_date = a.date <= b.date;
@@ -1010,7 +1018,7 @@ impl Config {
         }
 
         if let Some(c) = commits.last() {
-            if end != "origin/master" && !c.sha.starts_with(end) {
+            if !c.sha.starts_with(end) {
                 bail!("expected to end with {}, but ended with {}", end, c.sha);
             }
         }