chore: fix legacy git ref problems (#38827)

1. add correct "refs/heads" prefix to the branch name for commit graph
2. fix incorrect cache key in GetCommitGraphsCount
3. remove the "--" trim for the tag name, there is no security vulnerability, we never do so anywhere else

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

modules/base/tool.go

@@ -23,7 +23,7 @@ import (
 // EncodeSha256 string to sha256 hex value.
 func EncodeSha256(str string) string {
 	h := sha256.New()
-	_, _ = h.Write([]byte(str))
+	_, _ = h.Write(util.UnsafeStringToBytes(str))
 	return hex.EncodeToString(h.Sum(nil))
 }