Replace fs::absolute calls with AbsPathJoin calls

This adds better test coverage and will make it easier in #20744 to remove our dependency on the two-argument boost::filesystem::absolute() function which does not have a direct equivalent in C++17.

src/fs.h

@@ -21,6 +21,17 @@ namespace fs = boost::filesystem;
 namespace fsbridge {
     FILE *fopen(const fs::path& p, const char *mode);
 
+    /**
+     * Helper function for joining two paths
+     *
+     * @param[in] base  Base path
+     * @param[in] path  Path to combine with base
+     * @returns path unchanged if it is an absolute path, otherwise returns base joined with path. Returns base unchanged if path is empty.
+     * @pre  Base path must be absolute
+     * @post Returned path will always be absolute
+     */
+    fs::path AbsPathJoin(const fs::path& base, const fs::path& path);
+
     class FileLock
     {
     public: