Be silent even when using V=1

While --disable-silent-rules can be nice for developers on laptops,
many production build systems want verbose build logs so errors can be
tracked down more easily.  Production build systems tend to have more
sophisticated log filtering and analysis than terminal scrollback.

However, git.mk outputs 117 lines for each subdirectory.  For projects
which are still sadly using recursive make, the build log bloat can be
incredible.  gtk+ has over 47 subdirectories.  This works out to an
additional 5499 lines in the build log!

Since the .gitignore is really not critical for the actual build
process, this patch simply changes things so that we only output one
line, always.  Anyone who wants to debug it can easily remove the @
character locally.