Add inline diff in repos/pulls/files api

Add the inline diff on a per file basis to be compatible with GitHub

Example:

```
curl http://localhost:3000/api/v1/repos/cedstrom/test/pulls/1/files?token=[redacted]
```

```
[
    {
        "filename": ".gitignore",
        "status": "added",
        "additions": 2,
        "deletions": 0,
        "changes": 2,
        "html_url": "http://localhost:3000/cedstrom/test/src/commit/ce2c53223388eaaa026c25156b807e30acc05a56/.gitignore",
        "contents_url": "http://localhost:3000/api/v1/repos/cedstrom/test/contents/.gitignore?ref=ce2c53223388eaaa026c25156b807e30acc05a56",
        "raw_url": "http://localhost:3000/cedstrom/test/raw/commit/ce2c53223388eaaa026c25156b807e30acc05a56/.gitignore",
        "patch": "diff --git a/.gitignore b/.gitignore\n--- a/.gitignore\n+++ b/.gitignore\n@@ -0,1 +1,2 @@\n @@ -0,0 +1,2 @@\n++.env\n++.env.local\n\n"
    },
    {
        "filename": "README.md",
        "status": "changed",
        "additions": 1,
        "deletions": 0,
        "changes": 1,
        "html_url": "http://localhost:3000/cedstrom/test/src/commit/ce2c53223388eaaa026c25156b807e30acc05a56/README.md",
        "contents_url": "http://localhost:3000/api/v1/repos/cedstrom/test/contents/README.md?ref=ce2c53223388eaaa026c25156b807e30acc05a56",
        "raw_url": "http://localhost:3000/cedstrom/test/raw/commit/ce2c53223388eaaa026c25156b807e30acc05a56/README.md",
        "patch": "diff --git a/README.md b/README.md\n--- a/README.md\n+++ b/README.md\n@@ -1,2 +1,3 @@\n @@ -1,2 +1,3 @@\n  # test\n  \n++This is my first PR.\n\n"
    }
]
```

Author: chris-edstrom

modules/structs/pull.go

@@ -188,4 +188,6 @@ type ChangedFile struct {
 	ContentsURL string `json:"contents_url,omitempty"`
 	// The raw URL to download the file
 	RawURL string `json:"raw_url,omitempty"`
+	// The patch text for the file changes
+	Patch string `json:"patch,omitempty"`
 }

services/convert/convert.go

@@ -812,6 +812,7 @@ func ToChangedFile(f *gitdiff.DiffFile, repo *repo_model.Repository, commit stri
 		HTMLURL:          fmt.Sprint(repo.HTMLURL(), "/src/commit/", commit, "/", util.PathEscapeSegments(f.GetDiffFileName())),
 		ContentsURL:      fmt.Sprint(repo.APIURL(), "/contents/", util.PathEscapeSegments(f.GetDiffFileName()), "?ref=", commit),
 		RawURL:           fmt.Sprint(repo.HTMLURL(), "/raw/commit/", commit, "/", util.PathEscapeSegments(f.GetDiffFileName())),
+		Patch:            gitdiff.RenderUnifiedDiff(f),
 	}
 
 	return file

services/gitdiff/gitdiff.go

@@ -1058,6 +1058,88 @@ func createDiffFile(line string) *DiffFile {
 	return curFile
 }
 
+func RenderUnifiedDiff(file *DiffFile) string {
+	var sb strings.Builder
+
+	oldPath := file.OldName
+	if oldPath == "" {
+		oldPath = file.Name
+	}
+	newPath := file.Name
+
+	// File header
+	fmt.Fprintf(&sb, "diff --git a/%s b/%s\n", oldPath, newPath)
+	fmt.Fprintf(&sb, "--- a/%s\n", oldPath)
+	fmt.Fprintf(&sb, "+++ b/%s\n", newPath)
+
+	if file.IsBin {
+		return ("Binary files differ\n\n")
+	}
+
+	for _, section := range file.Sections {
+		// Compute hunk header
+		leftStart, leftCount := hunkRange(section, true)
+		rightStart, rightCount := hunkRange(section, false)
+		fmt.Fprintf(&sb, "@@ -%d,%d +%d,%d @@\n", leftStart, leftCount, rightStart, rightCount)
+
+		for _, line := range section.Lines {
+			prefix := " "
+			switch line.Type {
+			case DiffLineAdd:
+				prefix = "+"
+			case DiffLineDel:
+				prefix = "-"
+			}
+			sb.WriteString(prefix + line.Content)
+			if !strings.HasSuffix(line.Content, "\n") {
+				sb.WriteString("\n")
+			}
+		}
+	}
+	sb.WriteString("\n")
+
+	return sb.String()
+}
+
+// hunkRange calculates the start and length for either old or new file in a section
+func hunkRange(section *DiffSection, left bool) (start, count int) {
+	lines := section.Lines
+	if len(lines) == 0 {
+		return 0, 0
+	}
+
+	if left {
+		for _, l := range lines {
+			if l.LeftIdx > 0 {
+				start = l.LeftIdx
+				break
+			}
+		}
+		for _, l := range lines {
+			if l.LeftIdx > 0 {
+				count++
+			}
+		}
+	} else {
+		for _, l := range lines {
+			if l.RightIdx > 0 {
+				start = l.RightIdx
+				break
+			}
+		}
+		for _, l := range lines {
+			if l.RightIdx > 0 {
+				count++
+			}
+		}
+	}
+
+	if count == 0 {
+		count = 1
+	}
+	return start, count
+}
+
 func readFileName(rd *strings.Reader) (string, bool) {
 	ambiguity := false
 	var name string