To avoid that ugly Console window when calling \cmd\git.exe gui...
To avoid confusion with builtins, we need to move the code block
handling gitk (and now git-gui, too) to intercept before git-gui is
mistaken for a builtin.
Unfortunately, git-gui is in libexec/git-core/ while gitk is in bin/,
therefore we need slightly more adjustments than just moving and
augmenting the gitk case.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>