Merge different languages for language stats (#24900) (#24921)

Backport #24900

Fix #24896

If users set different languages by `linguist-language`, the `stats` map
could be: `java: 100, Java: 200`.

Language stats are stored as case-insensitive in database and there is a
unique key.

So, the different language names should be merged to one unique name:
`Java: 300`

(cherry picked from commit a83d597989bbfa6d7c10a1737bc40faa83224342)

modules/git/repo_language_stats_test.go

@@ -30,3 +30,17 @@ func TestRepository_GetLanguageStats(t *testing.T) {
 		"Java":   112,
 	}, stats)
 }
+
+func TestMergeLanguageStats(t *testing.T) {
+	assert.EqualValues(t, map[string]int64{
+		"PHP":    1,
+		"python": 10,
+		"JAVA":   700,
+	}, mergeLanguageStats(map[string]int64{
+		"PHP":    1,
+		"python": 10,
+		"Java":   100,
+		"java":   200,
+		"JAVA":   400,
+	}))
+}