Fix yet another bug with diff file names (#12771) (#12776)

Backport #12771

Following further testing it has become apparent that the diff line
cannot be used to determine filenames for diffs with any sort of predictability
the answer therefore is to use the other lines that are provided with a diff

Fix #12768

Signed-off-by: Andrew Thornton <art27@cantab.net>

services/gitdiff/gitdiff_test.go

@@ -6,6 +6,7 @@
 package gitdiff
 
 import (
+	"encoding/json"
 	"fmt"
 	"html/template"
 	"strings"
@@ -41,7 +42,145 @@ func TestDiffToHTML(t *testing.T) {
 	}, DiffLineDel))
 }
 
-func TestParsePatch(t *testing.T) {
+func TestParsePatch_singlefile(t *testing.T) {
+	type testcase struct {
+		name        string
+		gitdiff     string
+		wantErr     bool
+		addition    int
+		deletion    int
+		oldFilename string
+		filename    string
+	}
+
+	tests := []testcase{
+		{
+			name: "readme.md2readme.md",
+			gitdiff: `diff --git "a/README.md" "b/README.md"
+--- a/README.md
++++ b/README.md
+@@ -1,3 +1,6 @@
+ # gitea-github-migrator
++
++ Build Status
+- Latest Release
+ Docker Pulls
++ cut off
++ cut off
+`,
+			addition: 4,
+			deletion: 1,
+			filename: "README.md",
+		},
+		{
+			name: "A \\ B",
+			gitdiff: `diff --git "a/A \\ B" "b/A \\ B"
+--- "a/A \\ B"
++++ "b/A \\ B"
+@@ -1,3 +1,6 @@
+ # gitea-github-migrator
++
++ Build Status
+- Latest Release
+ Docker Pulls
++ cut off
++ cut off`,
+			addition: 4,
+			deletion: 1,
+			filename: "A \\ B",
+		},
+		{
+			name: "really weird filename",
+			gitdiff: `diff --git a/a b/file b/a a/file b/a b/file b/a a/file
+index d2186f1..f5c8ed2 100644
+--- a/a b/file b/a a/file	
++++ b/a b/file b/a a/file	
+@@ -1,3 +1,2 @@
+ Create a weird file.
+ 
+-and what does diff do here?
+\ No newline at end of file`,
+			addition:    0,
+			deletion:    1,
+			filename:    "a b/file b/a a/file",
+			oldFilename: "a b/file b/a a/file",
+		},
+		{
+			name: "delete file with blanks",
+			gitdiff: `diff --git a/file with blanks b/file with blanks
+deleted file mode 100644
+index 898651a..0000000
+--- a/file with blanks	
++++ /dev/null
+@@ -1,5 +0,0 @@
+-a blank file
+-
+-has a couple o line
+-
+-the 5th line is the last
+`,
+			addition: 0,
+			deletion: 5,
+			filename: "file with blanks",
+		},
+		{
+			name: "rename a—as",
+			gitdiff: `diff --git "a/\360\243\220\265b\342\200\240vs" "b/a\342\200\224as"
+similarity index 100%
+rename from "\360\243\220\265b\342\200\240vs"
+rename to "a\342\200\224as"
+`,
+			addition:    0,
+			deletion:    0,
+			oldFilename: "𣐵b†vs",
+			filename:    "a—as",
+		},
+		{
+			name: "rename with spaces",
+			gitdiff: `diff --git a/a b/file b/a a/file b/a b/a a/file b/b file
+similarity index 100%
+rename from a b/file b/a a/file
+rename to a b/a a/file b/b file
+`,
+			oldFilename: "a b/file b/a a/file",
+			filename:    "a b/a a/file b/b file",
+		},
+	}
+
+	for _, testcase := range tests {
+		t.Run(testcase.name, func(t *testing.T) {
+			got, err := ParsePatch(setting.Git.MaxGitDiffLines, setting.Git.MaxGitDiffLineCharacters, setting.Git.MaxGitDiffFiles, strings.NewReader(testcase.gitdiff))
+			if (err != nil) != testcase.wantErr {
+				t.Errorf("ParsePatch() error = %v, wantErr %v", err, testcase.wantErr)
+				return
+			}
+			gotMarshaled, _ := json.MarshalIndent(got, "  ", "  ")
+			if got.NumFiles() != 1 {
+				t.Errorf("ParsePath() did not receive 1 file:\n%s", string(gotMarshaled))
+				return
+			}
+			if got.TotalAddition != testcase.addition {
+				t.Errorf("ParsePath() does not have correct totalAddition %d, wanted %d", got.TotalAddition, testcase.addition)
+			}
+			if got.TotalDeletion != testcase.deletion {
+				t.Errorf("ParsePath() did not have correct totalDeletion %d, wanted %d", got.TotalDeletion, testcase.deletion)
+			}
+			file := got.Files[0]
+			if file.Addition != testcase.addition {
+				t.Errorf("ParsePath() does not have correct file addition %d, wanted %d", file.Addition, testcase.addition)
+			}
+			if file.Deletion != testcase.deletion {
+				t.Errorf("ParsePath() did not have correct file deletion %d, wanted %d", file.Deletion, testcase.deletion)
+			}
+			if file.OldName != testcase.oldFilename {
+				t.Errorf("ParsePath() did not have correct OldName %s, wanted %s", file.OldName, testcase.oldFilename)
+			}
+			if file.Name != testcase.filename {
+				t.Errorf("ParsePath() did not have correct Name %s, wanted %s", file.Name, testcase.filename)
+			}
+		})
+	}
+
 	var diff = `diff --git "a/README.md" "b/README.md"
 --- a/README.md
 +++ b/README.md