feat(api): support ref suffixes in compare (#38148)

Compare API requests with a `^` or `~N` revision suffix (for example
`compare/main...feature^`) were rejected with `400 Unsupported
comparison syntax: ref with suffix`. The fix resolves the suffix to a
commit before comparing, so `base...head^` and `~N` work on either side,
the same as git.

Only `^`/`~N` navigation is resolved. Pull request creation still
requires plain branch refs, and the web compare page keeps rejecting
suffixes since its branch selectors need separate UI work.

Closes #33943

modules/git/ref.go

@@ -228,13 +228,16 @@ func (ref RefName) RefWebLinkPath() string {
 	return string(refType) + "/" + util.PathEscapeSegments(ref.ShortName())
 }
 
-func ParseRefSuffix(ref string) (string, string) {
+func ParseRefSuffix(ref string) (refName, refSuffix string) {
 	// Partially support https://git-scm.com/docs/gitrevisions
-	if idx := strings.Index(ref, "@{"); idx != -1 {
-		return ref[:idx], ref[idx:]
+	suffixIdx := -1 // earliest suffix mark, so a combined suffix like "main~2^" stays intact
+	for _, mark := range []string{"@{", "^", "~"} {
+		if idx := strings.Index(ref, mark); idx != -1 && (suffixIdx == -1 || idx < suffixIdx) {
+			suffixIdx = idx
+		}
 	}
-	if idx := strings.Index(ref, "^"); idx != -1 {
-		return ref[:idx], ref[idx:]
+	if suffixIdx == -1 {
+		return ref, ""
 	}
-	return ref, ""
+	return ref[:suffixIdx], ref[suffixIdx:]
 }

modules/git/ref_test.go

@@ -37,3 +37,22 @@ func TestRefWebLinkPath(t *testing.T) {
 	assert.Equal(t, "tag/foo", RefName("refs/tags/foo").RefWebLinkPath())
 	assert.Equal(t, "commit/c0ffee", RefName("c0ffee").RefWebLinkPath())
 }
+
+func TestParseRefSuffix(t *testing.T) {
+	cases := []struct {
+		ref, name, suffix string
+	}{
+		{"main", "main", ""},
+		{"main^", "main", "^"},
+		{"main^2", "main", "^2"},
+		{"main~3", "main", "~3"},
+		{"main@{yesterday}", "main", "@{yesterday}"},
+		{"main~2^", "main", "~2^"},
+		{"main^~2", "main", "^~2"},
+	}
+	for _, c := range cases {
+		name, suffix := ParseRefSuffix(c.ref)
+		assert.Equal(t, c.name, name, "ref: %s", c.ref)
+		assert.Equal(t, c.suffix, suffix, "ref: %s", c.ref)
+	}
+}