setup.py: attempt to build and install hg.exe on Windows

Currently, packaging Mercurial on Windows will produce a
Scripts\hg Python script and a Scripts\hg.bat batch script. The
py2exe distribution contains a hg.exe which loads a Python
interpretter and invokes the "hg" Python script. Running a
exe directly has benefits over batch scripts because batch
scripts do things like muck around with command arguments.

This patch implements a custom "build_scripts" command which
attempts to build hg.exe on Windows. If hg.exe is built, it is
marked as a "script" file and installed into the Scripts\
directory on Windows. Since hg.exe is redundant and better than
hg.bat, if hg.exe is built, hg.bat is not installed.

Since some environments don't support compiling C programs,
we treat hg.exe as optional and catch failures building it. This
is not ideal. However, I reckon most Windows users will not be
installing Mercurial from source: they will get it from the MSI
installer or via `pip install Mercurial`, which will download a
wheel that has hg.exe in it. So, I don't think this is a big deal.

setup.py

@@ -79,11 +79,16 b' from distutils.dist import Distribution'
 from distutils.command.build import build
 from distutils.command.build_ext import build_ext
 from distutils.command.build_py import build_py
+from distutils.command.build_scripts import build_scripts
 from distutils.command.install_lib import install_lib
 from distutils.command.install_scripts import install_scripts
 from distutils.spawn import spawn, find_executable
 from distutils import file_util
-from distutils.errors import CCompilerError, DistutilsExecError
+from distutils.errors import (
+    CCompilerError,
+    DistutilsError,
+    DistutilsExecError,
+)
 from distutils.sysconfig import get_python_inc, get_config_var
 from distutils.version import StrictVersion
 
@@ -102,6 +107,7 b' elif sys.version_info[0] >= 3:'
 
 scripts = ['hg']
 if os.name == 'nt':
+    # We remove hg.bat if we are able to build hg.exe.
     scripts.append('contrib/win32/hg.bat')
 
 # simplified version of distutils.ccompiler.CCompiler.has_function
@@ -304,6 +310,31 b' class hgbuildext(build_ext):'
             log.warn("Failed to build optional extension '%s' (skipping)",
                      ext.name)
 
+class hgbuildscripts(build_scripts):
+    def run(self):
+        if os.name != 'nt':
+            return build_scripts.run(self)
+
+        exebuilt = False
+        try:
+            self.run_command('build_hgexe')
+            exebuilt = True
+        except (DistutilsError, CCompilerError):
+            log.warn('failed to build optional hg.exe')
+
+        if exebuilt:
+            # Copying hg.exe to the scripts build directory ensures it is
+            # installed by the install_scripts command.
+            hgexecommand = self.get_finalized_command('build_hgexe')
+            dest = os.path.join(self.build_dir, 'hg.exe')
+            self.mkpath(self.build_dir)
+            self.copy_file(hgexecommand.hgexepath, dest)
+
+            # Remove hg.bat because it is redundant with hg.exe.
+            self.scripts.remove('contrib/win32/hg.bat')
+
+        return build_scripts.run(self)
+
 class hgbuildpy(build_py):
     if convert2to3:
         fixer_names = sorted(set(getfixers("lib2to3.fixes") +
@@ -389,6 +420,11 b' class buildhgexe(build_ext):'
                                       libraries=[],
                                       output_dir=self.build_temp)
 
+    @property
+    def hgexepath(self):
+        dir = os.path.dirname(self.get_ext_fullpath('dummy'))
+        return os.path.join(self.build_temp, dir, 'hg.exe')
+
 class hginstalllib(install_lib):
     '''
     This is a specialization of install_lib that replaces the copy_file used
@@ -473,6 +509,7 b" cmdclass = {'build': hgbuild,"
             'build_mo': hgbuildmo,
             'build_ext': hgbuildext,
             'build_py': hgbuildpy,
+            'build_scripts': hgbuildscripts,
             'build_hgextindex': buildhgextindex,
             'install_lib': hginstalllib,
             'install_scripts': hginstallscripts,