Don't clobber global grepprg, or quickfix mappings

[ches]

'grepprg' is a buffer-local setting. We should only use the &l version, or else when we restore it after an :Ag command, we're disrupting the setting in other buffers.

It's surprising that 'grepprg' is local and global, and 'grepformat' is only global, so I guess we have no choice but to clobber it. Perhaps this has it's uses: suppose you have a C program with some tests in shell (or cram, etc., like ag!), and you always use ag for 'grepprg'. The same 'grepformat' can work project-wide, but you might have an autocommand that makes 'grepprg' use ag --shell for all files under the tests directory.

Anyway, it's kind of pathological, but we just shouldn't clobber a global setting when local can be used. I was working on something else when I made the small changes here, but that's going to warrant deeper review discussion when it's ready I think, so I went ahead and isolated these changes.

Update: I realized the <buffer> mappings were broken too, <buffer> must be given first or it doesn't work. Fix included.

[ches]

The last commit here was a little cleanup of a bug I left earlier. I'll squash the commits if this otherwise meets review approval.

Is there anything controversial here?

[losingkeys]

Wow, nice work. I have lots of questions (sorry in advance lol).

• I understand not clobbering the quickfix mappings, but aren't we resetting the grepprg after using it? Don't we want to do that so other plugins can have buffer-local mappings and we won't break those except when :Ag is running (short amounts of time)?

Some style comments/preferences (these are up for debate, but I'd prefer at least consistency if we choose something else):

• The plugins in $VIMRUNTIME/plugins/ all use g:loaded_<plugin-name> for their load once section, and I think I've seen this elsewhere. Mind renaming g:autoloaded_ag?
• As far as aligning = goes, it really only matters to me that things are consistent (you'll notice that they're not even before this pull, but maybe this is a good time to fix them). So a=b, a = b, or aligned with other =s in the same small block are all fine, but let's make them all match. Also formatting should be separate commits imo.

Other thoughts that might not pertain:

• It's odd that it just ignores <buffer>, maybe that's a bug in vim (though the docs mention it, I'm not sure why this limitation exists)
• Prefixing ex commands w/: doesn't really matter to me, as long as it works. So I'd say as long as things are consistent it's good. Mind checking if all our ex commands follow the same style?