LibDiff: Implement ability to parse a patch header

This is a somewhat naive implementation, but it is enough to parse a
simple unified patch header.

After parsing the patch header, the parser will be at the beginning of
the first hunks range, ready for that hunk to be parsed.

Userland/Libraries/LibDiff/Forward.h

@@ -8,8 +8,11 @@
 
 namespace Diff {
 
+enum class Format;
+
 class Parser;
 
+struct Header;
 struct Hunk;
 struct HunkLocation;
 struct Line;

Userland/Libraries/LibDiff/Hunks.cpp

@@ -57,6 +57,33 @@ bool Parser::consume_line_number(size_t& number)
     return true;
 }
 
+ErrorOr<Header> Parser::parse_header()
+{
+    Header header;
+
+    while (!is_eof()) {
+
+        if (consume_specific("+++ ")) {
+            header.new_file_path = TRY(String::from_utf8(consume_line()));
+            continue;
+        }
+
+        if (consume_specific("--- ")) {
+            header.old_file_path = TRY(String::from_utf8(consume_line()));
+            continue;
+        }
+
+        if (next_is("@@ ")) {
+            header.format = Format::Unified;
+            return header;
+        }
+
+        consume_line();
+    }
+
+    return Error::from_string_literal("Unable to find any patch");
+}
+
 ErrorOr<Vector<Hunk>> Parser::parse_hunks()
 {
     Vector<Hunk> hunks;

Userland/Libraries/LibDiff/Hunks.h

@@ -59,12 +59,26 @@ struct Hunk {
     Vector<Line> lines;
 };
 
+enum class Format {
+    Unified,
+    Unknown,
+};
+
+struct Header {
+    Format format { Format::Unknown };
+
+    String old_file_path;
+    String new_file_path;
+};
+
 class Parser : public GenericLexer {
 public:
     using GenericLexer::GenericLexer;
 
     ErrorOr<Vector<Hunk>> parse_hunks();
 
+    ErrorOr<Header> parse_header();
+
 private:
     Optional<HunkLocation> consume_unified_location();
     bool consume_line_number(size_t& number);