update comments in clang-format.py for python3 compatibility

[pseyfert]

Summary

D23319 introduced python3 compatibility to clang-format.py, this is however not reflected by the documentation in the comments at the beginning of the file, which show how to use the script with python2 in .vimrc. While the actual mapping a user might want to use may well differ from my suggestion, I think it's nice to show the python2 and python3 version, such that a user can pick from the suggestions instead of googeling the python3 replacement for :pyf which they might not be familiar with.

EDIT: picking reviewers according to https://llvm.org/docs/Phabricator.html#finding-potential-reviewers

[pseyfert]

ping?

[ilya-biryukov]

@MyDeveloperDay, added you on the assumption that you are interested in all things about clang-format. Hope the change is relevant, let me know if not.

[MyDeveloperDay]

I'm not python expert but as this is a comment I don't see any issue. LGTM

[MyDeveloperDay]

Do you need help landing this?

[pseyfert]

thanks for the ping. looking back at Code Reviews with Phabricator, the following applies to me: "If you do not have commit access, someone has to commit the change for you (with attribution)." so, yes I need help landing.

clang/tools/clang-format/clang-format.py

 # This file is a minimal clang-format vim-integration. To install:
 # - Change 'binary' if clang-format is not on the path (see below).
 # - Add to your .vimrc:
 #
-#   map <C-I> :pyf <path-to-this-file>/clang-format.py<cr>
-#   imap <C-I> <c-o>:pyf <path-to-this-file>/clang-format.py<cr>
+#   if has('python')
+#     map <C-I> :pyf <path-to-this-file>/clang-format.py<cr>
+#     imap <C-I> <c-o>:pyf <path-to-this-file>/clang-format.py<cr>
+#   elseif has('python3')
+#     map <C-I> :py3f <path-to-this-file>/clang-format.py<cr>
+#     imap <C-I> <c-o>:py3f <path-to-this-file>/clang-format.py<cr>
+#   endif
 #
-# The first line enables clang-format for NORMAL and VISUAL mode, the second
-# line adds support for INSERT mode. Change "C-I" to another binding if you
+# The if-elseif-endif conditional should pick either the python3 or python2 
+# integration depending on your vim setup.
+# 
+# The first mapping enables clang-format for NORMAL and VISUAL mode, the second
+# mapping adds support for INSERT mode. Change "C-I" to another binding if you
 # need clang-format on a different key (C-I stands for Ctrl+i).
 #
 # With this integration you can press the bound key and clang-format will
 # format the current line in NORMAL and INSERT mode or the selected region in
 # VISUAL mode. The line or region is extended to the next bigger syntactic
 # entity.
 #
 # You can also pass in the variable "l:lines" to choose the range for
 # formatting. This variable can either contain "<start line>:<end line>" or
 # "all" to format the full file. So, to format the full file, write a function
 # like:
 # :function FormatFile()
 # :  let l:lines="all"
-# :  pyf <path-to-this-file>/clang-format.py
+# :  if has('python')
+# :    pyf <path-to-this-file>/clang-format.py
+# :  elseif has('python3')
+# :    py3f <path-to-this-file>/clang-format.py
+# :  endif
 # :endfunction
 #
 # It operates on the current, potentially unsaved buffer and does not create
 # or save any files. To revert a formatting, just undo.
 from __future__ import absolute_import, division, print_function
 
 import difflib
 import json