[xb] Only write version.h when changed

xenia-build

@@ -252,6 +252,7 @@ def shell_call(command, throw_on_error=True, stdout_path=None, stderr_path=None,
 def generate_version_h():
     """Generates a build/version.h file that contains current git info.
     """
+    header_file = 'build/version.h'
     if git_is_repository():
         (branch_name, commit, commit_short) = git_get_head_info()
     else:
@@ -259,17 +260,25 @@ def generate_version_h():
         commit = ':(-dont-do-this'
         commit_short = ':('
 
-    contents = '''// Autogenerated by `xb premake`.
-  #ifndef GENERATED_VERSION_H_
-  #define GENERATED_VERSION_H_
-  #define XE_BUILD_BRANCH "%s"
-  #define XE_BUILD_COMMIT "%s"
-  #define XE_BUILD_COMMIT_SHORT "%s"
-  #define XE_BUILD_DATE __DATE__
-  #endif  // GENERATED_VERSION_H_
-  ''' % (branch_name, commit, commit_short)
-    with open('build/version.h', 'w') as f:
-        f.write(contents)
+    contents_new = '''// Autogenerated by `xb premake`.
+#ifndef GENERATED_VERSION_H_
+#define GENERATED_VERSION_H_
+#define XE_BUILD_BRANCH "{}"
+#define XE_BUILD_COMMIT "{}"
+#define XE_BUILD_COMMIT_SHORT "{}"
+#define XE_BUILD_DATE __DATE__
+#endif  // GENERATED_VERSION_H_
+'''.format(branch_name, commit, commit_short)
+
+    contents_old = None
+    if os.path.exists(header_file) and os.path.getsize(header_file) < 1024:
+        with open(header_file, 'r') as f:
+            contents_old = f.read()
+
+    if contents_old != contents_new:
+        with open(header_file, 'w') as f:
+            f.write(contents_new)
+
 
 def generate_source_class(path):
     header_path = '{}.h'.format(path)