LibArchive: Support writing paths up to 255 characters

The POSIX.1-1988 limit was 100, but with the ustar prefix field it's 255
(kinda).

Userland/Libraries/LibArchive/Tar.cpp

@@ -35,4 +35,22 @@ bool TarFileHeader::content_is_like_extended_header() const
     return type_flag() == TarFileType::ExtendedHeader || type_flag() == TarFileType::GlobalExtendedHeader;
 }
 
+void TarFileHeader::set_filename_and_prefix(StringView filename)
+{
+    // FIXME: Add support for extended tar headers for longer filenames.
+    VERIFY(filename.length() <= sizeof(m_filename) + sizeof(m_prefix));
+
+    if (filename.length() <= sizeof(m_filename)) {
+        set_prefix(""sv);
+        set_filename(filename);
+        return;
+    }
+
+    Optional<size_t> slash = filename.find('/', filename.length() - sizeof(m_filename));
+
+    VERIFY(slash.has_value());
+    set_prefix(filename.substring_view(0, slash.value() + 1));
+    set_filename(filename.substring_view(slash.value() + 1));
+}
+
 }

Userland/Libraries/LibArchive/Tar.h

@@ -130,6 +130,8 @@ public:
 
     bool content_is_like_extended_header() const;
 
+    void set_filename_and_prefix(StringView filename);
+
 private:
     char m_filename[100] { 0 };
     char m_mode[8] { 0 };

Userland/Libraries/LibArchive/TarStream.cpp

@@ -133,7 +133,7 @@ void TarOutputStream::add_directory(String const& path, mode_t mode)
     VERIFY(!m_finished);
     TarFileHeader header {};
     header.set_size(0);
-    header.set_filename(String::formatted("{}/", path)); // Old tar implementations assume directory names end with a /
+    header.set_filename_and_prefix(String::formatted("{}/", path)); // Old tar implementations assume directory names end with a /
     header.set_type_flag(TarFileType::Directory);
     header.set_mode(mode);
     header.set_magic(gnu_magic);
@@ -149,7 +149,7 @@ void TarOutputStream::add_file(String const& path, mode_t mode, ReadonlyBytes by
     VERIFY(!m_finished);
     TarFileHeader header {};
     header.set_size(bytes.size());
-    header.set_filename(path);
+    header.set_filename_and_prefix(path);
     header.set_type_flag(TarFileType::NormalFile);
     header.set_mode(mode);
     header.set_magic(gnu_magic);