Keep permissions of the saved files intact.

Previously the permission were destroyed by the newly created
temporary file -- which is later used as new saved file.
Tested on symlinks as well but only on Linux.

src/Buffer.cpp

@@ -1443,6 +1443,9 @@ bool Buffer::save() const
 
 	FileName savefile(tempfile->name());
 	LYXERR(Debug::FILES, "Saving to " << savefile.absFileName());
+	if (!savefile.clonePermissions(fileName()))
+		LYXERR0("Failed to clone the permission from " << fileName().absFileName() << " to " << savefile.absFileName());
+
 	if (!writeFile(savefile))
 		return false;
 

src/support/FileName.cpp

@@ -288,6 +288,13 @@ bool FileName::changePermission(unsigned long int mode) const
 	return true;
 }
 
+bool FileName::clonePermissions(FileName const & source)
+{
+	QFile fin(toqstr(source.absFileName()));
+	QFile f(toqstr(absFileName()));
+
+	return f.setPermissions(fin.permissions());
+}
 
 string FileName::toFilesystemEncoding() const
 {

src/support/FileName.h

@@ -150,6 +150,8 @@ public:
 	/// support this.
 	/// \return true on success.
 	bool changePermission(unsigned long int mode) const;
+	// sets permission based on the file given as an argument
+	bool clonePermissions(FileName const & target);
 
 	/// remove pointed directory and all contents.
 	/// \return true on success.