Automatically add macros for C in git patch

For c/cpp code I want to add all my newly added changes inside a macro and removed changes in the else part.

e.g. for the below patch

diff --git a/test.c b/test.c
index a35d23a..5ee498a 100644
--- a/test.c
+++ b/test.c
@@ -1,6 +1,6 @@
 #include <stdio.h>

 int main() {
-    printf("Old line");
+    printf("New line");
     return 0;
 }

modified to

diff --git a/test.c b/test.c
index a35d23a..668d2e0 100644
--- a/test.c
+++ b/test.c
@@ -1,6 +1,10 @@
 #include <stdio.h>

 int main() {
+#ifdef MODIFIED
+    printf("New line");
+#else
     printf("Old line");
+#endif
     return 0;
 }

Or

diff --git a/test.c b/test.c
index a35d23a..d05f7d1 100644
--- a/test.c
+++ b/test.c
@@ -1,6 +1,11 @@
 #include <stdio.h>

 int main() {
+#ifdef MODIFIED
+    printf("New line");
+#endif
+#ifndef MODIFIED
     printf("Old line");
+#endif
     return 0;
 }

I tried searching if there is some inbuilt tool for doing this. Unfortunately could not find one.

Can someone please help. TIA

This is built into diff, at least on Linux and macOS (it is not in the POSIX specification for diff). From https://linux.die.net/man/1/diff:

-D NAME --ifdef=NAME

Output merged file to show '#ifdef NAME' diffs.

For a POSIX diff without this feature, diff -U 99999999 FileA FileB will produce the entire contents of reasonable files, and then a simple script can insert a #if defined !Name where - starts in the first column, a #else where a - changes to a +, a #if defined Name where + starts without a preceding -, and a #endif where + or - ends, along with removing the headers and the leading character of each line (a space for unchanged lines). That leaves the groups of lines reversed from what you show, so it has the old lines first and then the new lines. If that is not acceptable, you could swap the order of the files on the diff command line and change the senses of - and +.