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)

services/gitdiff/gitdiff_excerpt.go

@@ -0,0 +1,121 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gitdiff
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"html/template"
+	"io"
+
+	"code.gitea.io/gitea/modules/setting"
+)
+
+type BlobExcerptOptions struct {
+	LastLeft      int
+	LastRight     int
+	LeftIndex     int
+	RightIndex    int
+	LeftHunkSize  int
+	RightHunkSize int
+	Direction     string
+	Language      string
+}
+
+func fillExcerptLines(section *DiffSection, filePath string, reader io.Reader, lang string, idxLeft, idxRight, chunkSize int) error {
+	buf := &bytes.Buffer{}
+	scanner := bufio.NewScanner(reader)
+	var diffLines []*DiffLine
+	for line := 0; line < idxRight+chunkSize; line++ {
+		if ok := scanner.Scan(); !ok {
+			break
+		}
+		lineText := scanner.Text()
+		if buf.Len()+len(lineText) < int(setting.UI.MaxDisplayFileSize) {
+			buf.WriteString(lineText)
+			buf.WriteByte('\n')
+		}
+		if line < idxRight {
+			continue
+		}
+		diffLine := &DiffLine{
+			LeftIdx:  idxLeft + (line - idxRight) + 1,
+			RightIdx: line + 1,
+			Type:     DiffLinePlain,
+			Content:  " " + lineText,
+		}
+		diffLines = append(diffLines, diffLine)
+	}
+	if err := scanner.Err(); err != nil {
+		return fmt.Errorf("fillExcerptLines scan: %w", err)
+	}
+	section.Lines = diffLines
+	// DiffLinePlain always uses right lines
+	section.highlightedRightLines.value = highlightCodeLines(filePath, lang, []*DiffSection{section}, false /* right */, buf.Bytes())
+	return nil
+}
+
+func BuildBlobExcerptDiffSection(filePath string, reader io.Reader, opts BlobExcerptOptions) (*DiffSection, error) {
+	lastLeft, lastRight, idxLeft, idxRight := opts.LastLeft, opts.LastRight, opts.LeftIndex, opts.RightIndex
+	leftHunkSize, rightHunkSize, direction := opts.LeftHunkSize, opts.RightHunkSize, opts.Direction
+	language := opts.Language
+
+	chunkSize := BlobExcerptChunkSize
+	section := &DiffSection{
+		language:              &diffVarMutable[string]{value: language},
+		highlightedLeftLines:  &diffVarMutable[map[int]template.HTML]{},
+		highlightedRightLines: &diffVarMutable[map[int]template.HTML]{},
+		FileName:              filePath,
+	}
+	var err error
+	if direction == "up" && (idxLeft-lastLeft) > chunkSize {
+		idxLeft -= chunkSize
+		idxRight -= chunkSize
+		leftHunkSize += chunkSize
+		rightHunkSize += chunkSize
+		err = fillExcerptLines(section, filePath, reader, language, idxLeft-1, idxRight-1, chunkSize)
+	} else if direction == "down" && (idxLeft-lastLeft) > chunkSize {
+		err = fillExcerptLines(section, filePath, reader, language, lastLeft, lastRight, chunkSize)
+		lastLeft += chunkSize
+		lastRight += chunkSize
+	} else {
+		offset := -1
+		if direction == "down" {
+			offset = 0
+		}
+		err = fillExcerptLines(section, filePath, reader, language, lastLeft, lastRight, idxRight-lastRight+offset)
+		leftHunkSize = 0
+		rightHunkSize = 0
+		idxLeft = lastLeft
+		idxRight = lastRight
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	newLineSection := &DiffLine{
+		Type: DiffLineSection,
+		SectionInfo: &DiffLineSectionInfo{
+			language:      &diffVarMutable[string]{value: opts.Language},
+			Path:          filePath,
+			LastLeftIdx:   lastLeft,
+			LastRightIdx:  lastRight,
+			LeftIdx:       idxLeft,
+			RightIdx:      idxRight,
+			LeftHunkSize:  leftHunkSize,
+			RightHunkSize: rightHunkSize,
+		},
+	}
+	if newLineSection.GetExpandDirection() != "" {
+		newLineSection.Content = fmt.Sprintf("@@ -%d,%d +%d,%d @@\n", idxLeft, leftHunkSize, idxRight, rightHunkSize)
+		switch direction {
+		case "up":
+			section.Lines = append([]*DiffLine{newLineSection}, section.Lines...)
+		case "down":
+			section.Lines = append(section.Lines, newLineSection)
+		}
+	}
+	return section, nil
+}