Allow dependency's rev to refer to a ref by name

Author: dtolnay

src/cargo/sources/git/utils.rs

@@ -799,13 +799,17 @@ pub fn fetch(
             refspecs.push(String::from("HEAD:refs/remotes/origin/HEAD"));
         }
 
-        // For `rev` dependencies we don't know what the rev will point to. To
-        // handle this situation we fetch all branches and tags, and then we
-        // pray it's somewhere in there.
-        GitReference::Rev(_) => {
-            refspecs.push(String::from("refs/heads/*:refs/remotes/origin/*"));
-            refspecs.push(String::from("HEAD:refs/remotes/origin/HEAD"));
-            tags = true;
+        GitReference::Rev(rev) => {
+            if rev.starts_with("refs/") {
+                refspecs.push(format!("{0}:{0}", rev));
+            } else {
+                // We don't know what the rev will point to. To handle this
+                // situation we fetch all branches and tags, and then we pray
+                // it's somewhere in there.
+                refspecs.push(String::from("refs/heads/*:refs/remotes/origin/*"));
+                refspecs.push(String::from("HEAD:refs/remotes/origin/HEAD"));
+                tags = true;
+            }
         }
     }