blob: GetBlobContent: reduce allocations (#8223)

See #8222 for context.

## git.Blob.NewTruncatedReader

This introduce a new `NewTruncatedReader` method to return a blob-reader which silently truncates when the limit is reached (io.EOF will be returned).
Since the actual size is also returned `GetBlobContent` can pre-allocate a `[]byte` of the full-size (min of the asked size and the actual size) and call `io.ReadFull(rc, buf)` (instead of `util.ReadWithLimit(dataRc, int(limit))` which is convoluted and not used anywhere else).

### Tests

- I added test coverage for Go changes...
  - [x] in their respective `*_test.go` for unit tests.

### Documentation

- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [x] I do not want this change to show in the release notes.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8223
Reviewed-by: Lucas <sclu1034@noreply.codeberg.org>
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: oliverpool <git@olivier.pfad.fr>
Co-committed-by: oliverpool <git@olivier.pfad.fr>

modules/git/blob_test.go

@@ -35,6 +35,43 @@ func TestBlob_Data(t *testing.T) {
 	assert.Equal(t, output, string(data))
 }
 
+func TestBlob_GetBlobContent(t *testing.T) {
+	bareRepo1Path := filepath.Join(testReposDir, "repo1_bare")
+	repo, err := openRepositoryWithDefaultContext(bareRepo1Path)
+	require.NoError(t, err)
+
+	defer repo.Close()
+
+	testBlob, err := repo.GetBlob("6c493ff740f9380390d5c9ddef4af18697ac9375")
+	require.NoError(t, err)
+
+	r, err := testBlob.GetBlobContent(100)
+	require.NoError(t, err)
+	require.Equal(t, "file2\n", r)
+
+	r, err = testBlob.GetBlobContent(-1)
+	require.NoError(t, err)
+	require.Empty(t, r)
+
+	r, err = testBlob.GetBlobContent(4)
+	require.NoError(t, err)
+	require.Equal(t, "file", r)
+
+	r, err = testBlob.GetBlobContent(6)
+	require.NoError(t, err)
+	require.Equal(t, "file2\n", r)
+
+	t.Run("non-existing blob", func(t *testing.T) {
+		inexistingBlob, err := repo.GetBlob("00003ff740f9380390d5c9ddef4af18690000000")
+		require.NoError(t, err)
+
+		r, err := inexistingBlob.GetBlobContent(100)
+		require.Error(t, err)
+		require.IsType(t, ErrNotExist{}, err)
+		require.Empty(t, r)
+	})
+}
+
 func Benchmark_Blob_Data(b *testing.B) {
 	bareRepo1Path := filepath.Join(testReposDir, "repo1_bare")
 	repo, err := openRepositoryWithDefaultContext(bareRepo1Path)