osutil: export a "getfstype" method

This patch exports the "getfstype" method. So we can use it to enable
hardlinks for known safe filesystems.

The patch was tested manually via debugshell on a Linux system.
"mercurial.osutil.getfstype" works as expected. It's hard to mount
filesystem on user-space easily. I will add a test for real hardlink support
to indirectly test this patch, after turning on real hardlinks support for
certain whitelisted filesystems.

mercurial/osutil.c

@@ -1079,6 +1079,20 b' const char *getfstype(const char *path) '
 	/* End of Linux filesystems */
 	return NULL;
 }
+
+static PyObject *pygetfstype(PyObject *self, PyObject *args)
+{
+	const char *path = NULL;
+	if (!PyArg_ParseTuple(args, "s", &path))
+		return NULL;
+
+	const char *type = getfstype(path);
+	if (type == NULL)
+		Py_RETURN_NONE;
+
+	PyObject *result = Py_BuildValue("s", type);
+	return result;
+}
 #endif /* def HAVE_STATFS */
 
 #endif /* ndef _WIN32 */
@@ -1257,6 +1271,10 b' static PyMethodDef methods[] = {'
 	{"setprocname", (PyCFunction)setprocname, METH_VARARGS,
 	 "set process title (best-effort)\n"},
 #endif
+#ifdef HAVE_STATFS
+	{"getfstype", (PyCFunction)pygetfstype, METH_VARARGS,
+	 "get filesystem type (best-effort)\n"},
+#endif
 #endif /* ndef _WIN32 */
 #ifdef __APPLE__
 	{