LibDiff+patch: Support multiple patches in a single patch file

Multiple patches may be concatenated in the same patch file, such as git
commits which are changing multiple files at the same time. To handle
this, parse each patch in order in the patch file, and apply each patch
sequentially.

To determine whether we are at the end of a patch (and not just parsing
another hunk) the parser will look for a leading '@@ ' after every hunk.
If that is found, there is another hunk. Otherwise, we must be at the
end of this patch.

Tests/Utilities/TestPatch.cpp

@@ -165,3 +165,25 @@ TEST_CASE(add_file_from_scratch)
 
     EXPECT_FILE_EQ(MUST(String::formatted("{}/file_to_add", s_test_dir)), "Hello, friends!\n");
 }
+
+TEST_CASE(two_patches_in_single_patch_file)
+{
+    PatchSetup setup;
+
+    auto patch = R"(
+--- /dev/null
++++ a/first_file_to_add
+@@ -0,0 +1 @@
++Hello, friends!
+--- /dev/null
++++ a/second_file_to_add
+@@ -0,0 +1 @@
++Hello, friends!
+)"sv;
+
+    run_patch({}, patch, "patching file first_file_to_add\n"
+                         "patching file second_file_to_add\n"sv);
+
+    EXPECT_FILE_EQ(MUST(String::formatted("{}/first_file_to_add", s_test_dir)), "Hello, friends!\n");
+    EXPECT_FILE_EQ(MUST(String::formatted("{}/second_file_to_add", s_test_dir)), "Hello, friends!\n");
+}