Use OSTYPE instead of uname whenever possible for better speed. (#5496)

plugins/gitfast/git-completion.bash

@@ -2771,6 +2771,6 @@ __git_complete gitk __gitk_main
 # when the user has tab-completed the executable name and consequently
 # included the '.exe' suffix.
 #
-if [ Cygwin = "$(uname -o 2>/dev/null)" ]; then
+if [[ "$OSTYPE" = cygwin* ]]; then
 __git_complete git.exe __git_main
 fi