Various PKGBUILD simplifications/cleanups:

- Get rid of $_gitname in place of cloning the repo to $pkgname
- Get rid of unneeded $srcdir usage; all package functions start out in
  $srcdir anyways
- Removed manual build dir creation; cmake does this automatically & the
  NVim build instructions no longer recommend this.
- Bracketed two variables for consistency
- Removed unneeded quotes
- Removed empty conflicts()
- Removed gawk & sh from depends(); gawk is part of base-devel and is
  assumed, regardless of what namcap says (it's frequently wrong), and
  sh is already pulled in by multiple packages in base-devel so is not
  needed
- Removed -DUSE_BUNDLED=Off; during building cmake complains that it is
  an unused flag, and diff -ur confirms no difference in pkg.tar.xz
  contents with and without it (besides the sha256sum of /usr/bin/nvim,
  which is only because the binary is built non-deterministically).
- Source is now cloned directly using git as opposed to HTTPS for speed,

  See the following
  http://git-scm.com/book/en/Git-on-the-Server-The-Protocols

Signed-off-by: Florian Hahn <flo@fhahn.com>