s/type -f/type -t/ to check for __gitcomp function

In `lib/git-hub.d/init`, to check for the availability of a `__gitcomp`
Bash function, use `type -t __gitcomp` rather than `type -f __gitcomp`.

From my limited testing, `type -t` appears to give the desired output,
whereas `type -f` would not.
Merge pull request #142 from 8573/8573/fix/shell-env-init-script/type-t-not-type-f/1

s/type -f/type -t/ to check for __gitcomp function