git-gui: Correct gitk installation location

git-gui: Correct gitk installation location

The master Makefile in git.git installs gitk into bindir, not
gitexecdir, which means gitk is located as a sibling of the git
wrapper and not as though it were a git helper tool.

We can also avoid some Tcl concat operations by letting eval do
all of the heavy lifting; we have two proper Tcl lists ($cmd and
$revs) that we are joining together and $revs is currently never
an empty list.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>