fastexport: rework newline logic

Newlines should only be added when otherwise the stream would look weird
without them (on blobs), therefore they are the exception.

Flip the logic so they are added, not skipped.

hgext/fastexport.py

@@ -69,10 +69,10 b' def convert_to_git_ref(branch):'
     return b"refs/heads/" + branch
 
 
-def write_data(buf, data, skip_newline):
+def write_data(buf, data, add_newline=False):
     buf.append(b"data %d\n" % len(data))
     buf.append(data)
-    if not skip_newline or data[-1:] != b"\n":
+    if add_newline or data[-1:] != b"\n":
         buf.append(b"\n")
 
 
@@ -103,7 +103,7 b' def export_commit(ui, repo, rev, marks, '
             marks[filerev] = mark
             data = filectx.data()
             buf = [b"blob\n", b"mark :%d\n" % mark]
-            write_data(buf, data, False)
+            write_data(buf, data, True)
             ui.write(*buf, keepprogressbar=True)
             del buf
 
@@ -122,7 +122,7 b' def export_commit(ui, repo, rev, marks, '
             convert_to_git_date(ctx.date()),
         ),
     ]
-    write_data(buf, ctx.description(), True)
+    write_data(buf, ctx.description())
     if parents:
         buf.append(b"from :%d\n" % marks[parents[0].hex()])
     if len(parents) == 2: