Use full-file highlighting for diff sections (#36561)

* Fix #35252
* Fix #35999
* Improve diff rendering, don't add unnecessary "added"/"removed" tags for a full-line change
* Also fix a "space trimming" bug in #36539 and add tests
* Use chroma "SQL" lexer instead of "MySQL" to workaround a bug (35999)

modules/highlight/highlight_test.go

@@ -108,6 +108,12 @@ c=2
 			),
 			lexerName: "Python",
 		},
+		{
+			name:      "test.sql",
+			code:      "--\nSELECT",
+			want:      []template.HTML{"<span class=\"c1\">--\n</span>", `<span class="k">SELECT</span>`},
+			lexerName: "SQL",
+		},
 	}
 
 	for _, tt := range tests {