assemble module: fix insertion of newlines when not needed.

This builds on GH-6359 and changes the logic so that a newline is only inserted between fragments if the previous fragment does not end with a newline.

library/files/assemble

@@ -102,17 +102,22 @@ def assemble_from_fragments(src_path, delimiter=None, compiled_regexp=None):
     tmpfd, temp_path = tempfile.mkstemp()
     tmp = os.fdopen(tmpfd,'w')
     delimit_me = False
+    add_newline = False
 
     for f in sorted(os.listdir(src_path)):
         if compiled_regexp and not compiled_regexp.search(f):
             continue
         fragment = "%s/%s" % (src_path, f)
+        if not os.path.isfile(fragment):
+            continue
+        fragment_content = file(fragment).read()
 
-        # delimiters should only appear between fragments
-        if delimit_me:
-            # always put a newline between fragments
+        # always put a newline between fragments if the previous fragment didn't end with a newline.
+        if add_newline:
             tmp.write('\n')
 
+        # delimiters should only appear between fragments
+        if delimit_me:
             if delimiter:
                 # un-escape anything like newlines
                 delimiter = delimiter.decode('unicode-escape')
@@ -122,9 +127,12 @@ def assemble_from_fragments(src_path, delimiter=None, compiled_regexp=None):
                 if delimiter[-1] != '\n':
                     tmp.write('\n')
 
-        if os.path.isfile(fragment):
-            tmp.write(file(fragment).read())
+        tmp.write(fragment_content)
         delimit_me = True
+        if fragment_content.endswith('\n'):
+            add_newline = False
+        else:
+            add_newline = True
 
     tmp.close()
     return temp_path