gix-revision: Parse revisions with the @ character

Fixes #802

Author: lthms

gix-revision/src/spec/parse/function.rs

@@ -344,8 +344,15 @@ where
     {
         let mut cursor = input;
         let mut ofs = 0;
-        while let Some((pos, b)) = cursor.iter().enumerate().find(|(_, b)| {
-            if b"@~^:.".contains(b) {
+        while let Some((pos, b)) = cursor.iter().enumerate().find(|(pos, b)| {
+            if **b == b'@' {
+                cursor.get(pos + 1) == Some(&b'{')
+                    || cursor
+                        .get(pos + 1)
+                        .and_then(|b| Some(b"~^:.".contains(b)))
+                        .unwrap_or(false)
+                    || (*pos == 0 && cursor.get(1) == None)
+            } else if b"~^:.".contains(b) {
                 true
             } else {
                 if let Some(num) = consecutive_hex_chars.as_mut() {

gix-revision/tests/spec/parse/anchor/refnames.rs

@@ -10,15 +10,10 @@ fn at_by_itself_is_shortcut_for_head() {
 }
 
 #[test]
-fn multiple_ats_are_invalid_but_may_cause_callbacks() {
-    let err = try_parse("@@").unwrap_err();
-    assert!(matches!(err, spec::parse::Error::AtNeedsCurlyBrackets {input} if input == "@"));
-}
-
-#[test]
-fn lonely_at_after_ref_is_invalid() {
-    let err = try_parse("HEAD@").unwrap_err();
-    assert!(matches!(err, spec::parse::Error::AtNeedsCurlyBrackets {input} if input == "@"));
+fn at_is_allowed() {
+    let rec = parse("a@b");
+    assert!(rec.kind.is_none());
+    assert_eq!(rec.get_ref(0), "a@b");
 }
 
 #[test]