Correctly compute the changed files based on each git hook

* Also let `jt lint fast` (without extra argument) work and assume everything changed.
* Notably avoids running checks when just rewording a commit and using pre-commit.
* Git post-commit hooks seem unfortunately to not be able to see that no files
  changed when just rewording a commit or rebasing.

Author: eregon

tool/hooks/lint-check.sh

@@ -8,4 +8,10 @@
 #   cp tool/hooks/lint-check.sh .git/hooks/pre-push
 # The choice is yours.
 
-exec tool/jt.rb lint fast
+filename=$(basename "${BASH_SOURCE[0]}")
+
+case "$filename" in
+  pre-commit)   exec tool/jt.rb lint fast HEAD ;;
+  post-commit)  exec tool/jt.rb lint fast HEAD^ ;;
+  pre-push)     exec tool/jt.rb lint fast origin/master ;;
+esac

tool/jt.rb

@@ -2386,14 +2386,11 @@ def format_specializations_arguments
 
   def lint(*args)
     fast = args.first == 'fast'
-    if fast
-      changed_files = `git diff --cached --name-only` # Only staged files in the git index
-      if changed_files.empty? # post-commit hook
-        changed_files = `git diff --cached --name-only HEAD^`
-      end
-      raise 'Could not list changed files' if changed_files.empty?
+    args.shift if fast
+
+    if fast and compare_to = args.shift
+      changed_files = `git diff --cached --name-only #{compare_to}`
       exts_changed = changed_files.lines.map { |f| File.extname(f.strip) }.uniq
-      raise 'Could not list changed file extensions' if exts_changed.empty?
       changed = -> ext { exts_changed.include?(ext) }
     else
       changed = -> _ext { true }