Make "meson help" and "meson help <subcommand>" do the expected thing.

docs/markdown/snippets/improved-help.md

@@ -0,0 +1,6 @@
+## "meson help" now shows command line help
+
+Command line parsing is now less surprising. "meson help" is now
+equivalent to "meson --help" and "meson help <subcommand>" is
+equivalent to "meson <subcommand> --help", instead of creating a build
+directory called "help" in these cases.

mesonbuild/mesonmain.py

@@ -284,6 +284,13 @@ def run(original_args, mainfile=None):
         # First check if we want to run a subcommand.
         cmd_name = args[0]
         remaining_args = args[1:]
+        # "help" is a special case: Since printing of the help may be
+        # delegated to a subcommand, we edit cmd_name before executing
+        # the rest of the logic here.
+        if cmd_name == 'help':
+            remaining_args += ['--help']
+            args = remaining_args
+            cmd_name = args[0]
         if cmd_name == 'test':
             return mtest.run(remaining_args)
         elif cmd_name == 'setup':