cmd: on windows, we will now fallback to using git.cmd, as the default installation puts it into the path, instead of git.exe. Of course we warn about the performance penalty that goes with it. Fortunately, using a wrapped git.exe works when creating pipes to the process, admittedly I was a bit surprised by this.

fixes gitpython-developers#26

Author: Byron

git/cmd.py

@@ -48,11 +48,15 @@ class Git(LazyMixin):
 	# The size in bytes read from stdout when copying git's output to another stream
 	max_chunk_size = 1024*64
 
+	git_exec_name = "git"			# default that should work on linux and windows
+	git_exec_name_win = "git.cmd"	# alternate command name, windows only
+	
 	# Enables debugging of GitPython's git commands
 	GIT_PYTHON_TRACE = os.environ.get("GIT_PYTHON_TRACE", False)
 
 	# Provide the full path to the git executable. Otherwise it assumes git is in the path
-	GIT_PYTHON_GIT_EXECUTABLE = os.environ.get("GIT_PYTHON_GIT_EXECUTABLE", 'git')
+	_git_exec_env_var = "GIT_PYTHON_GIT_EXECUTABLE"
+	GIT_PYTHON_GIT_EXECUTABLE = os.environ.get(_git_exec_env_var, git_exec_name)
 
 
 	class AutoInterrupt(object):
@@ -449,11 +453,40 @@ def _call_process(self, method, *args, **kwargs):
 
 		ext_args = self.__unpack_args([a for a in args if a is not None])
 		args = opt_args + ext_args
-
-		call = [self.GIT_PYTHON_GIT_EXECUTABLE, dashify(method)]
-		call.extend(args)
-
-		return self.execute(call, **_kwargs)
+		
+		def make_call():
+			call = [self.GIT_PYTHON_GIT_EXECUTABLE, dashify(method)]
+			call.extend(args)
+			return call
+		#END utility to recreate call after changes
+		
+		if sys.platform == 'win32':
+			try:
+				try:
+					return self.execute(make_call(), **_kwargs)
+				except WindowsError:
+					# did we switch to git.cmd already, or was it changed from default ? permanently fail
+					if self.GIT_PYTHON_GIT_EXECUTABLE != self.git_exec_name:
+						raise
+					#END handle overridden variable
+					type(self).GIT_PYTHON_GIT_EXECUTABLE = self.git_exec_name_win
+					call = [self.GIT_PYTHON_GIT_EXECUTABLE] + list(args)
+					
+					try:
+						return self.execute(make_call(), **_kwargs)
+					finally:
+						import warnings
+						msg = "WARNING: Automatically switched to use git.cmd as git executable, which reduces performance by ~70%."
+						msg += "Its recommended to put git.exe into the PATH or to set the %s environment variable to the executable's location" % self._git_exec_env_var 
+						warnings.warn(msg)
+					#END print of warning
+				#END catch first failure
+			except WindowsError:
+				raise WindowsError("The system cannot find or execute the file at %r" % self.GIT_PYTHON_GIT_EXECUTABLE)
+			#END provide better error message
+		else:
+			return self.execute(make_call(), **_kwargs)
+		#END handle windows default installation
 
 	def _parse_object_header(self, header_line):
 		"""