goto: use existing window

This add `jedi_vim.edit_buffer`, factored out of `new_buffer`, and
`goto` uses it.

Ref: davidhalter#408

jedi_vim.py

@@ -228,7 +228,7 @@ def goto(is_definition=False, is_related_name=False, no_output=False):
                                    % d.desc_with_module)
             else:
                 if d.module_path != vim.current.buffer.name:
-                    result = new_buffer(d.module_path)
+                    result = edit_buffer(d.module_path)
                     if not result:
                         return
                 vim.current.window.cursor = d.line, d.column
@@ -521,7 +521,7 @@ def py_import():
             echo_highlight('%s is a builtin module.' % import_path)
         else:
             cmd_args = ' '.join([a.replace(' ', '\\ ') for a in args])
-            new_buffer(completion.module_path, cmd_args)
+            edit_buffer(completion.module_path, cmd_args)
 
 
 @catch_and_print_exceptions
@@ -539,8 +539,22 @@ def py_import_completions():
     vim.command("return '%s'" % '\n'.join(comps))
 
 
+@catch_and_print_exceptions
+def edit_buffer(path, options=''):
+    """Edit the given path in the current window."""
+    if vim_eval("!&hidden && &modified") == '1':
+        if vim_eval("bufname('%')") is None:
+            echo_highlight('Cannot open a new buffer, use `:set hidden` or save your buffer')
+            return False
+        else:
+            vim_command('w')
+    vim_command('edit %s %s' % (options, escape_file_path(path)))
+    fix_buffer_options()
+
+
 @catch_and_print_exceptions
 def new_buffer(path, options=''):
+    """Edit the given path in a new window."""
     # options are what you can to edit the edit options
     if vim_eval('g:jedi#use_tabs_not_buffers') == '1':
         _tabnew(path, options)
@@ -560,19 +574,18 @@ def new_buffer(path, options=''):
         else:
             vim_command(split_options[user_split_option] + " %s" % path)
     else:
-        if vim_eval("!&hidden && &modified") == '1':
-            if vim_eval("bufname('%')") is None:
-                echo_highlight('Cannot open a new buffer, use `:set hidden` or save your buffer')
-                return False
-            else:
-                vim_command('w')
-        vim_command('edit %s %s' % (options, escape_file_path(path)))
+        edit_buffer(path, options)
+
+    fix_buffer_options()
+    return True
+
+
+def fix_buffer_options():
     # sometimes syntax is being disabled and the filetype not set.
     if vim_eval('!exists("g:syntax_on")') == '1':
         vim_command('syntax enable')
     if vim_eval("&filetype != 'python'") == '1':
         vim_command('set filetype=python')
-    return True
 
 
 @catch_and_print_exceptions