Print correct command in help message

Taking mconf for instance:
before:
  $ meson configure --help
  usage: meson [-h] [-D SETS] [--clearcache] [directory [directory ...]]
after:
  $ meson configure --help
  usage: meson configure [-h] [-D SETS] [--clearcache] [directory [directory ...]]

mesonbuild/mconf.py

@@ -17,7 +17,7 @@ import pickle
 import argparse
 from . import coredata, mesonlib
 
-parser = argparse.ArgumentParser()
+parser = argparse.ArgumentParser(prog='meson configure')
 
 parser.add_argument('-D', action='append', default=[], dest='sets',
                     help='Set an option to the given value.')

mesonbuild/mesonmain.py

@@ -24,7 +24,7 @@ from .mesonlib import MesonException
 from .wrap import WrapMode, wraptool
 
 
-parser = argparse.ArgumentParser()
+parser = argparse.ArgumentParser(prog='meson')
 
 default_warning = '1'
 

mesonbuild/mintro.py

@@ -25,7 +25,7 @@ import argparse
 import sys, os
 import pathlib
 
-parser = argparse.ArgumentParser()
+parser = argparse.ArgumentParser(prog='meson introspect')
 parser.add_argument('--targets', action='store_true', dest='list_targets', default=False,
                     help='List top level targets.')
 parser.add_argument('--installed', action='store_true', dest='list_installed', default=False,

mesonbuild/mtest.py

@@ -58,7 +58,7 @@ def determine_worker_count():
             num_workers = 1
     return num_workers
 
-parser = argparse.ArgumentParser()
+parser = argparse.ArgumentParser(prog='meson test')
 parser.add_argument('--repeat', default=1, dest='repeat', type=int,
                     help='Number of times to run the tests.')
 parser.add_argument('--no-rebuild', default=False, action='store_true',

mesonbuild/rewriter.py

@@ -29,7 +29,7 @@ from mesonbuild import mlog
 import sys, traceback
 import argparse
 
-parser = argparse.ArgumentParser()
+parser = argparse.ArgumentParser(prog='meson rewrite')
 
 parser.add_argument('--sourcedir', default='.',
                     help='Path to source directory.')