Do not use code location for Gitlab fingerprints.

Using code location when creating hash for Gitlab fingerprints
makes the codequality reports in Gitlab very unstable.
Any change of position would trigger both a "fixed" message, and
a "new problem detected" message in Gitlab, making it very noisy.

crates/ruff/src/message/gitlab.rs

@@ -1,13 +1,12 @@
 use std::collections::hash_map::DefaultHasher;
+use std::collections::HashSet;
 use std::hash::{Hash, Hasher};
 use std::io::Write;
 
 use serde::ser::SerializeSeq;
 use serde::{Serialize, Serializer};
 use serde_json::json;
 
-use ruff_source_file::SourceLocation;
-
 use crate::fs::{relativize_path, relativize_path_to};
 use crate::message::{Emitter, EmitterContext, Message};
 use crate::registry::AsRule;
@@ -58,6 +57,7 @@ impl Serialize for SerializedMessages<'_> {
         S: Serializer,
     {
         let mut s = serializer.serialize_seq(Some(self.messages.len()))?;
+        let mut fingerprints = HashSet::<String>::new();
 
         for message in self.messages {
             let start_location = message.compute_start_location();
@@ -82,10 +82,16 @@ impl Serialize for SerializedMessages<'_> {
                 |project_dir| relativize_path_to(message.filename(), project_dir),
             );
 
+            let mut message_fingerprint = fingerprint(message, "".to_string());
+            while fingerprints.contains(&message_fingerprint) {
+                message_fingerprint = fingerprint(message, message_fingerprint);
+            }
+            fingerprints.insert(message_fingerprint.to_string());
+
             let value = json!({
                 "description": format!("({}) {}", message.kind.rule().noqa_code(), message.kind.body),
                 "severity": "major",
-                "fingerprint": fingerprint(message, &start_location, &end_location),
+                "fingerprint": message_fingerprint,
                 "location": {
                     "path": path,
                     "lines": lines
@@ -102,8 +108,7 @@ impl Serialize for SerializedMessages<'_> {
 /// Generate a unique fingerprint to identify a violation.
 fn fingerprint(
     message: &Message,
-    start_location: &SourceLocation,
-    end_location: &SourceLocation,
+    salt: String,
 ) -> String {
     let Message {
         kind,
@@ -116,8 +121,7 @@ fn fingerprint(
     let mut hasher = DefaultHasher::new();
 
     kind.rule().hash(&mut hasher);
-    start_location.hash(&mut hasher);
-    end_location.hash(&mut hasher);
+    salt.hash(&mut hasher);
     file.name().hash(&mut hasher);
 
     format!("{:x}", hasher.finish())