Support worktrees in lefthook/security harness

Our scripts/security-harness did not work with GitLab repositories using
worktrees because:
- It relies on lefthook, which only added support for worktrees in 0.7.5
- It makes an assumption about the location of the pre-push hook path

This change bumps the lefthook gem to 0.7.5 to fix the first problem,
and uses a more robust way to get the pre-push hook path to fix the
second problem.

Gemfile.lock

@@ -690,7 +690,7 @@ GEM
       rest-client (~> 2.0)
     launchy (2.5.0)
       addressable (~> 2.7)
-    lefthook (0.7.2)
+    lefthook (0.7.5)
     letter_opener (1.7.0)
       launchy (~> 2.2)
     letter_opener_web (1.4.0)

scripts/security-harness

@@ -4,6 +4,7 @@
 
 require 'digest'
 require 'fileutils'
+require 'open3'
 
 if ENV['NO_COLOR']
   SHELL_RED    = ''
@@ -18,7 +19,7 @@ else
 end
 
 LEFTHOOK_GLOBAL_CONFIG_PATH = File.expand_path("../lefthook.yml", __dir__)
-HOOK_PATH = File.expand_path("../.git/hooks/pre-push", __dir__)
+HOOK_PATH = Open3.capture3("git rev-parse --path-format=absolute --git-path hooks/pre-push")[0].strip
 HOOK_DATA = <<~HOOK
   #!/usr/bin/env bash