file: Refactor COPY/INSTALL relative path handling

Delay conversion to absolute path until the last moment.

Source/cmFileCommand.cxx

@@ -1314,13 +1314,7 @@ bool cmFileCopier::CheckValue(std::string const& arg)
 {
   switch (this->Doing) {
     case DoingFiles:
-      if (arg.empty() || cmSystemTools::FileIsFullPath(arg.c_str())) {
-        this->Files.push_back(arg);
-      } else {
-        std::string file = this->Makefile->GetCurrentSourceDirectory();
-        file += "/" + arg;
-        this->Files.push_back(file);
-      }
+      this->Files.push_back(arg);
       break;
     case DoingDestination:
       if (arg.empty() || cmSystemTools::FileIsFullPath(arg.c_str())) {
@@ -1390,11 +1384,20 @@ bool cmFileCopier::Run(std::vector<std::string> const& args)
     return false;
   }
 
-  std::vector<std::string> const& files = this->Files;
-  for (std::vector<std::string>::size_type i = 0; i < files.size(); ++i) {
+  for (std::vector<std::string>::const_iterator i = this->Files.begin();
+       i != this->Files.end(); ++i) {
+    std::string file;
+    if (!i->empty() && !cmSystemTools::FileIsFullPath(*i)) {
+      file = this->Makefile->GetCurrentSourceDirectory();
+      file += "/";
+      file += *i;
+    } else {
+      file = *i;
+    }
+
     // Split the input file into its directory and name components.
     std::vector<std::string> fromPathComponents;
-    cmSystemTools::SplitPath(files[i], fromPathComponents);
+    cmSystemTools::SplitPath(file, fromPathComponents);
     std::string fromName = *(fromPathComponents.end() - 1);
     std::string fromDir = cmSystemTools::JoinPath(
       fromPathComponents.begin(), fromPathComponents.end() - 1);