brew.rb: don't exit when failing to set devcmdrun.

It's not necessary to make this a hard failure so don't (and this makes
it more consistent with `brew.sh`).

Fixes #1462.

Library/Homebrew/brew.rb

@@ -65,8 +65,8 @@ begin
     unless internal_cmd
       internal_cmd = require? HOMEBREW_LIBRARY_PATH.join("dev-cmd", cmd)
       if internal_cmd && !ARGV.homebrew_developer?
-        safe_system "git", "config", "--file=#{HOMEBREW_REPOSITORY}/.git/config",
-                                     "--replace-all", "homebrew.devcmdrun", "true"
+        system "git", "config", "--file=#{HOMEBREW_REPOSITORY}/.git/config",
+                                "--replace-all", "homebrew.devcmdrun", "true"
         ENV["HOMEBREW_DEV_CMD_RUN"] = "1"
       end
     end