Fix undo and partial format

[Ram-Z]

This should fix the undo issue as well as improve partial formatting.

It works by sending only the selected lines to clang-format to be formatted, instead of the entire file. We then remember the indentation (because it gets lost during format) and reapply it to the formatted selection.

This was originally committed on Ram-Z@19b4346, but a change in circumstances led me to forget about it since. I've rebased that commit onto the current master. Sorry for the delay.

Fix: #8

[Ram-Z]

Didn't realize there were tests.

So... clang_format#format(line1, line2) now only returns the part between line1 and line2 which has been formatted. This is not what the tests expect. I'll have a quick look at fixing these.

[Ram-Z]

I've spent more time than I care to on this now. See the last commit msg for details on what is failing.

I'm happy to have another look at fixing the tests after I know that you are interested in merging this in.

[Ram-Z]

@rhysd Any comments on this?

[rhysd]

As long as looking code, this would break the code when formatting only line 4 of below code

#include <iostream>
int main()
{
    std::cout << "Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the \"Software\"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:";
}

I think slicing the formatted code does not work.

[Ram-Z]

OK... I made some wrong assumption about how > would work.

With this code the above does work:

let indent = indent(a:line1)
silent exe a:line1.','a:line2.'delete _'

call setreg('g', formatted, 'V')
silent put! g
silent exe "'[,']s/^/".repeat(' ', indent)."/"

but it has other problems when noexpandtab is set for example, nothing that can't be worked around.

The problem with replacing the entire file is that you are always replacing everything in the file even though you only format a single line. I think that slicing is the way to go. I would however prefer if clang-format would simply keep the starting indentation of the snippet when it formats it.