Detect file header, when parsing Hunk (#53)

The version before was failing to identify next file header (without extended
lines before it), while was parsing Hunk.  Hunk reader identify line as a part
of the hunk, if it starts with , + or -. Because of this, file header starting
with --- , was also identified as part of the hunk.

Author: sofiia-tesliuk

diff/diff_test.go

@@ -630,6 +630,25 @@ func TestParseMultiFileDiffHeaders(t *testing.T) {
 				},
 			},
 		},
+		{
+			filename: "sample_multi_file_without_extended.diff",
+			wantDiffs: []*FileDiff{
+				{
+					OrigName: "source_1_a/file_1.txt",
+					OrigTime: nil,
+					NewName:  "source_1_c/file_1.txt",
+					NewTime:  nil,
+					Extended: nil,
+				},
+				{
+					OrigName: "source_1_a/file_2.txt",
+					OrigTime: nil,
+					NewName:  "source_1_c/file_2.txt",
+					NewTime:  nil,
+					Extended: nil,
+				},
+			},
+		},
 	}
 	for _, test := range tests {
 		diffData, err := ioutil.ReadFile(filepath.Join("testdata", test.filename))

diff/parse.go

@@ -308,7 +308,7 @@ func (r *FileDiffReader) readOneFileHeader(prefix []byte) (filename string, time
 type OverflowError string
 
 func (e OverflowError) Error() string {
-	return fmt.Sprintf("overflowed into next file: %s", e)
+	return fmt.Sprintf("overflowed into next file: %s", string(e))
 }
 
 // ReadExtendedHeaders reads the extended header lines, if any, from a
@@ -531,7 +531,7 @@ func (r *HunksReader) ReadHunk() (*Hunk, error) {
 				return r.hunk, nil
 			}
 
-			if len(line) >= 1 && !linePrefix(line[0]) {
+			if len(line) >= 1 && (!linePrefix(line[0]) || bytes.HasPrefix(line, []byte("--- "))) {
 				// Bad hunk header line. If we're reading a multi-file
 				// diff, this may be the end of the current
 				// file. Return a "rich" error that lets our caller

diff/testdata/sample_multi_file_without_extended.diff

@@ -0,0 +1,31 @@
+--- source_1_a/file_1.txt
++++ source_1_c/file_1.txt
+@@ -1,11 +1,13 @@
+ Text generated by www.randomtextgenerator.com
+
++
+ In to am attended desirous raptures declared diverted confined at.
++In to are desirous declared diverted confined at.
++In to am attended raptures declared diverted confined at.
++In to am attended desirous confined at.
+ Collected instantly remaining up certainly to necessary as.
+ Over walk dull into son boy door went new.
+ At or happiness commanded daughters as.
+ Is handsome an declared at received in extended vicinity subjects.
+-Is handsome received in extended vicinity subjects.
+-Is handsome an declared at vicinity subjects.
+ Into miss on he over been late pain an.
+ Only week bore boy what fat case left use.
+--- source_1_a/file_2.txt
++++ source_1_c/file_2.txt
+@@ -4,8 +4,8 @@
+ Still round match we to.
+ Frankness pronounce daughters remainder extensive has but.
++Happiness cordially one determine concluded fat.
+ Plenty season beyond by hardly giving of.
+-Consulted or acuteness dejection an smallness if.
+-Outward general passage another as it.
+ Very his are come man walk one next.
+ Delighted prevailed supported too not remainder perpetual who furnished.
+ Nay affronting bed projection compliment instrument.
++Still round match we to here.