Report short sources as conflicts during apply (#63)

Previously, this returned an io.ErrUnexpectedEOF. This is not wrong, in
that we did unexpectedly hit the end of the input file, but it is vague
and implies a possible library bug rather than a problem with the patch
or the input. This condition is really a conflict, as the changes
described by the patch are not compatible with the state of the input.

gitdiff/apply_test.go

@@ -31,14 +31,14 @@ func TestApplyTextFragment(t *testing.T) {
 				Src:   "text_fragment_error.src",
 				Patch: "text_fragment_error_short_src_before.patch",
 			},
-			Err: io.ErrUnexpectedEOF,
+			Err: &Conflict{},
 		},
 		"errorShortSrc": {
 			Files: applyFiles{
 				Src:   "text_fragment_error.src",
 				Patch: "text_fragment_error_short_src.patch",
 			},
-			Err: io.ErrUnexpectedEOF,
+			Err: &Conflict{},
 		},
 		"errorContextConflict": {
 			Files: applyFiles{

gitdiff/apply_text.go

@@ -1,6 +1,7 @@
 package gitdiff
 
 import (
+	"errors"
 	"io"
 )
 
@@ -85,6 +86,11 @@ func (a *TextApplier) ApplyFragment(f *TextFragment) error {
 	preimage := make([][]byte, fragEnd-start)
 	n, err := a.lineSrc.ReadLinesAt(preimage, start)
 	if err != nil {
+		// an EOF indicates that source file is shorter than the patch expects,
+		// which should be reported as a conflict rather than a generic error
+		if errors.Is(err, io.EOF) {
+			err = &Conflict{"src has fewer lines than required by fragment"}
+		}
 		return applyError(err, lineNum(start+int64(n)))
 	}