Add path checks in #send_file and #get_git_dir

The Grack::Server class should reject invalid paths supplied by the user
using the regular expressions in line 10-23 of lib/grack/server.rb. This
change adds some defense in depth against invalid paths.
Merge branch 'guard_send_file' into 'master'

Add path checks in #send_file and #get_git_dir

The Grack::Server class should reject invalid paths supplied by the user
using the regular expressions in line 10-23 of lib/grack/server.rb. This
change adds some defense in depth against invalid paths.

See merge request !3
Merge branch 'master' of github.com:gitlabhq/grack