fix: guard checkDiffFile against mode-only diffs with empty OrigName/NewName

Mode-only diffs (e.g. chmod changes) and some binary diffs produce no
--- / +++ file headers in the unified diff output. The sourcegraph/go-diff
parser therefore returns empty OrigName and NewName strings for those
FileDiff entries. checkDiffFile assumed at least one '/' was always present
(from the 'a/' or 'b/' prefix) and unconditionally accessed index [1] after
SplitN, causing a runtime panic:

  panic: runtime error: index out of range [1] with length 1
      diff/diff.go:44

Guard the slice length before indexing and skip entries with no file headers
— there is no content to scan in mode-only or header-less diffs.

Author: ktersius

diff/diff.go

@@ -41,6 +41,11 @@ func checkDiffFile(parsedDiff *diff.FileDiff, workspace string) (filePath string
 	// If file is being renamed we don't want to check it for flags.
 	parsedFileA := strings.SplitN(parsedDiff.OrigName, "/", 2)
 	parsedFileB := strings.SplitN(parsedDiff.NewName, "/", 2)
+	// Mode-only diffs (e.g. chmod changes) and some binary diffs emit no --- / +++ headers.
+	// go-diff returns empty OrigName/NewName for these entries; there is no content to scan.
+	if len(parsedFileA) < 2 || len(parsedFileB) < 2 {
+		return "", true
+	}
 	fullPathToA := workspace + "/" + parsedFileA[1]
 	fullPathToB := workspace + "/" + parsedFileB[1]
 	info, err := os.Stat(fullPathToB)
@@ -90,4 +95,4 @@ func ProcessDiffs(matcher lsearch.Matcher, contents []byte, builder *refs.Refere
 			break
 		}
 	}
-}
+}