Improve tmux integration when changing buffers or panes when vim was
invoked by another process so that seamless navigation works when the
`vim$TMUX_PANE` window option has been set, not only when the pane's
command matches a simple regex.
Previously the `autocmd` to replace the date on the first line if `Last
change: ` was present resulted in the cursor being moved in non `help`
filetypes due to the substitution failing. Now the substitution is only
attempted if the current file is a `help` file.
Add `autocmd` to automatically expand the `_template` snippet on the
`BufNewFile` event, if the expansion fails because the `_template`
snippet does not exist for the current `filetype` undo the changes. Do
the same thing when the `filetype` changes to `help`, since these are
`text` files to begin with the first `autocmd` has no effect.
For all filetype's highlight conflict markers with the Todo highlight
group and leave trailing text as before. This makes conflict markers
very obvious, in C/C++ where I have operator highlighting conflict
markers are not longer highlighted as operators.
