Report short sources as conflicts during apply

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.

Author: bluekeyes

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)))
 	}