string_util: Remove unnecessary std::string instance in TabsToSpaces()

We can just use the variant of std::string's replace() function that can
replace an occurrence with N copies of the same character, eliminating
the need to allocate a std::string containing a buffer of spaces.

src/common/string_util.h

@@ -57,7 +57,7 @@ static bool TryParse(const std::string& str, N* const output) {
         return false;
 }
 
-std::string TabsToSpaces(int tab_size, const std::string& in);
+std::string TabsToSpaces(int tab_size, std::string in);
 
 void SplitString(const std::string& str, char delim, std::vector<std::string>& output);