Fix submodule always building from HEAD instead of recorded commit (#415)

Remove `git submodule update --remote` which overwrites the submodule
checkout with the latest remote HEAD, ignoring the commit recorded in
the superproject.

I noticed that builds produced by CI did not match the tagged version.
The root cause turned out to be that after `git submodule update --init
--depth 20` correctly checks out the commit pinned by the superproject,
the subsequent `git submodule update --remote` immediately overwrites it
with whatever is at the tip of the remote tracking branch. This means
every build was effectively building from vim HEAD rather than the
version the tag intended to release.

Author: mattn

appveyor.bat

@@ -194,7 +194,6 @@ echo TAG_NAME: %TAG_NAME%
 
 @rem Get Vim source code
 git submodule update --init --depth 20
-git submodule update --remote
 
 @rem Apply experimental patches
 pushd vim