git: Set sysconfdir during the build

Configuration files should be in /usr/local/etc (or equivalent), not in
the cellar, where git put them before.

Closes Homebrew/homebrew#22082.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/git.rb

@@ -47,6 +47,7 @@ class Git < Formula
     ENV['NO_GETTEXT'] = '1' unless build.with? 'gettext'
 
     system "make", "prefix=#{prefix}",
+                   "sysconfdir=#{etc}",
                    "CC=#{ENV.cc}",
                    "CFLAGS=#{ENV.cflags}",
                    "LDFLAGS=#{ENV.ldflags}",