Avoid redundant file copy when the file is newly allocated

When buildAppendFilePart is called with chunk data as the source we need
to copy it as the lifetimes are decoupled; however for changed files,
we've already allocated a vector for it so it's a waste to copy it as we
can move instead.

src/build.cpp

@@ -600,7 +600,7 @@ BuildContext* buildStart(Output* output, const char* path, unsigned int fileCoun
 	return context.release();
 }
 
-void buildAppendFilePart(BuildContext* context, const char* path, unsigned int startLine, const char* data, size_t dataSize, uint64_t timeStamp, uint64_t fileSize)
+static void appendFilePart(BuildContext* context, const char* path, unsigned int startLine, const char* data, size_t dataSize, uint64_t timeStamp, uint64_t fileSize, std::vector<char>* dataSource)
 {
 	if (!context->pendingFiles.empty() && context->pendingFiles.back().name == path)
 	{
@@ -623,7 +623,7 @@ void buildAppendFilePart(BuildContext* context, const char* path, unsigned int s
 		file.startLine = startLine;
 		file.timeStamp = timeStamp;
 		file.fileSize = fileSize;
-		file.contents = std::vector<char>(data, data + dataSize);
+		file.contents = dataSource ? std::move(*dataSource) : std::vector<char>(data, data + dataSize);
 
 		context->pendingFiles.emplace_back(file);
 		context->pendingSize += dataSize;
@@ -639,6 +639,11 @@ void buildAppendFilePart(BuildContext* context, const char* path, unsigned int s
 	}
 }
 
+void buildAppendFilePart(BuildContext* context, const char* path, unsigned int startLine, const char* data, size_t dataSize, uint64_t timeStamp, uint64_t fileSize)
+{
+	appendFilePart(context, path, startLine, data, dataSize, timeStamp, fileSize, nullptr);
+}
+
 bool buildAppendFile(BuildContext* context, const char* path, uint64_t timeStamp, uint64_t fileSize)
 {
 	FileStream in(path, "rb");
@@ -652,7 +657,7 @@ bool buildAppendFile(BuildContext* context, const char* path, uint64_t timeStamp
 	{
 		std::vector<char> contents = convertToUTF8(readFile(in));
 
-		buildAppendFilePart(context, path, 0, contents.empty() ? 0 : &contents[0], contents.size(), timeStamp, fileSize);
+		appendFilePart(context, path, 0, contents.empty() ? 0 : &contents[0], contents.size(), timeStamp, fileSize, &contents);
 
 		return true;
 	}