Fix fixup's false filename detection in hunks containing dashed lines (#5004)

### PR Description

The existing diff parser incorrectly treated subsequent lines beginning
with `---` as filename headers while processing hunks. This caused
corruption when dashed lines appeared within diffs themselves.

Restrict filename detection to only occur between hunks.

This repository https://github.com/abobov/lazygit-fixup-issue
demonstrates the issue:

- clone repository
- run `git reset --soft HEAD~`
- try `<c-f>` Find base commit for fixup 
- `fatal: no such path ...` error

pkg/gui/controllers/helpers/fixup_helper.go

@@ -194,7 +194,7 @@ func parseDiff(diff string) ([]*hunk, []*hunk) {
 		if strings.HasPrefix(line, "diff --git") {
 			finishHunk()
 			currentHunk = nil
-		} else if strings.HasPrefix(line, "--- ") {
+		} else if currentHunk == nil && strings.HasPrefix(line, "--- ") {
 			// For some reason, the line ends with a tab character if the file
 			// name contains spaces
 			filename = strings.TrimRight(line[6:], "\t")

pkg/gui/controllers/helpers/fixup_helper_test.go

@@ -78,6 +78,26 @@ index 9ce8efb33..fb5e469e7 100644
 				},
 			},
 		},
+		{
+			name: "hunk with dashed lines",
+			diff: `
+diff --git a/file1.txt b/file1.txt
+index 9ce8efb33..fb5e469e7 100644
+--- a/file1.txt
++++ b/file1.txt
+@@ -3,1 +3,1 @@
+--- xxx
++-- yyy
+`,
+			expectedDeletedLineHunks: []*hunk{
+				{
+					filename:     "file1.txt",
+					startLineIdx: 3,
+					numLines:     1,
+				},
+			},
+			expectedAddedLineHunks: []*hunk{},
+		},
 		{
 			name: "several hunks in different files",
 			diff: `