Do not overwrite template files, if they exist (#516)

This patch enables to place files in the debian folder of the upstream repository.
Currently it's possible to place additional files into that folder. However if the file is present in the bloom template set (like rules.em) that file is overwritten.

See #507.

Author: mistoll

bloom/generators/debian/generator.py

@@ -147,16 +147,16 @@ def __place_template_folder(group, src, dst, gbp=False):
             if not os.path.exists(dst):
                 os.makedirs(dst)
             if os.path.exists(template_dst):
-                debug("Removing existing file '{0}'".format(template_dst))
-                os.remove(template_dst)
-            with io.open(template_dst, 'w', encoding='utf-8') as f:
-                if not isinstance(template, str):
-                    template = template.decode('utf-8')
-                # Python 2 API needs a `unicode` not a utf-8 string.
-                elif sys.version_info.major == 2:
-                    template = template.decode('utf-8')
-                f.write(template)
-            shutil.copystat(template_abs_path, template_dst)
+                debug("Not overwriting existing file '{0}'".format(template_dst))
+            else:
+                with io.open(template_dst, 'w', encoding='utf-8') as f:
+                    if not isinstance(template, str):
+                        template = template.decode('utf-8')
+                    # Python 2 API needs a `unicode` not a utf-8 string.
+                    elif sys.version_info.major == 2:
+                        template = template.decode('utf-8')
+                    f.write(template)
+                shutil.copystat(template_abs_path, template_dst)
 
 
 def place_template_files(path, build_type, gbp=False):
@@ -838,7 +838,9 @@ def place_template_files(self, build_type, debian_dir='debian'):
         place_template_files('.', build_type, gbp=True)
         # Commit results
         execute_command('git add ' + debian_dir)
-        execute_command('git commit -m "Placing debian template files"')
+        _, has_files, _ = execute_command('git diff --cached --name-only', return_io=True)
+        if has_files:
+            execute_command('git commit -m "Placing debian template files"')
 
     def get_releaser_history(self):
         # Assumes that this is called in the target branch