buildbot: retrieve short commit hash properly

Instead of trying to manually parse the output of 'git show' which can be different across different git configurations, properly use the 'git rev-parse' command that is intended for this purpose.

util/buildbot/buildwin32.sh

@@ -81,7 +81,7 @@ else
 	[ -d minetest ] && (cd minetest && git pull) || (git clone https://github.com/minetest/minetest)
 fi
 cd minetest
-git_hash=`git show | head -c14 | tail -c7`
+git_hash=$(git rev-parse --short HEAD)
 
 # Get minetest_game
 cd games

util/buildbot/buildwin64.sh

@@ -76,7 +76,7 @@ else
 	[ -d minetest ] && (cd minetest && git pull) || (git clone https://github.com/minetest/minetest)
 fi
 cd minetest
-git_hash=`git show | head -c14 | tail -c7`
+git_hash=$(git rev-parse --short HEAD)
 
 # Get minetest_game
 cd games