docs: add necessary headers to Documentation/MFOW.txt

The tutorial in Documentation/MyFirstObjectWalk.txt
contains the functions trace_printf(), oid_to_hex(),
and pp_commit_easy(), and struct oidset, which are used
without any hint of where they are defined. When the provided
code is compiled, the compiler returns an error, stating that
the functions and the struct are used before declaration. Therefore,include
necessary header files (the ones which have no mentions in the tutorial).

Signed-off-by: Vinayak Dev <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: vinayakdsci

Documentation/MyFirstObjectWalk.txt

@@ -41,6 +41,7 @@ Open up a new file `builtin/walken.c` and set up the command handler:
  */
 
 #include "builtin.h"
+#include "trace.h"
 
 int cmd_walken(int argc, const char **argv, const char *prefix)
 {
@@ -49,12 +50,13 @@ int cmd_walken(int argc, const char **argv, const char *prefix)
 }
 ----
 
-NOTE: `trace_printf()` differs from `printf()` in that it can be turned on or
-off at runtime. For the purposes of this tutorial, we will write `walken` as
-though it is intended for use as a "plumbing" command: that is, a command which
-is used primarily in scripts, rather than interactively by humans (a "porcelain"
-command). So we will send our debug output to `trace_printf()` instead. When
-running, enable trace output by setting the environment variable `GIT_TRACE`.
+NOTE: `trace_printf()`, defined in `trace.h`, differs from `printf()` in
+that it can be turned on or off at runtime. For the purposes of this
+tutorial, we will write `walken` as though it is intended for use as
+a "plumbing" command: that is, a command which is used primarily in
+scripts, rather than interactively by humans (a "porcelain" command).
+So we will send our debug output to `trace_printf()` instead.
+When running, enable trace output by setting the environment variable `GIT_TRACE`.
 
 Add usage text and `-h` handling, like all subcommands should consistently do
 (our test suite will notice and complain if you fail to do so).
@@ -341,6 +343,10 @@ the walk loop below the `prepare_revision_walk()` call within your
 `walken_commit_walk()`:
 
 ----
+#include "pretty.h"
+
+...
+
 static void walken_commit_walk(struct rev_info *rev)
 {
 	struct commit *commit;
@@ -754,6 +760,10 @@ reachable objects are walked in order to populate the list.
 First, add the `struct oidset` and related items we will use to iterate it:
 
 ----
+#include "oidset.h"
+
+...
+
 static void walken_object_walk(
 	...
 
@@ -805,6 +815,10 @@ just walks of commits. First, we'll make our handlers chattier - modify
 go:
 
 ----
+#include "hex.h"
+
+...
+
 static void walken_show_commit(struct commit *cmt, void *buf)
 {
 	trace_printf("commit: %s\n", oid_to_hex(&cmt->object.oid));