git-gui: Quiet our installation process

git-gui: Quiet our installation process

Alex Riesen wanted a quieter installation process for git and its
contained git-gui.  His earlier patch to do this failed to work
properly when V=1, and didn't really give a great indication of
what the installation was doing.

These rules are a little bit on the messy side, as each of our
install actions is composed of at least two variables, but in the
V=1 case the text is identical to what we had before, while in the
non-V=1 case we use some more complex rules to show the interesting
details, and hide the less interesting bits.

We now can also set QUIET= (nothing) to see the rules that are used
when V= (nothing), so we can debug those too if we have to.  This is
actually a side-effect of how we insert the @ into the rules we use
for the "lists of things", like our builtins or our library files.

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