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+builtin API
+Adding a new built-in
+There are 4 things to do to add a bulit-in command implementation to
+. Define the implementation of the built-in command `foo` with
+ signature:
+ int cmd_foo(int argc, const char **argv, const char *prefix);
+. Add the external declaration for the function to `builtin.h`.
+. Add the command to `commands[]` table in `handle_internal_command()`,
+ defined in `git.c`. The entry should look like:
+ { "foo", cmd_foo, <options> },
+ where options is the bitwise-or of:
+ Make sure there is a git directory to work on, and if there is a
+ work tree, chdir to the top of it if the command was invoked
+ in a subdirectory. If there is no work tree, no chdir() is
+ done.
+ If the standard output is connected to a tty, spawn a pager and
+ feed our output to it.
+. Add `builtin-foo.o` to `BUILTIN_OBJS` in `Makefile`.
+Additionally, if `foo` is a new command, there are 3 more things to do:
+. Add tests to `t/` directory.
+. Write documentation in `Documentation/git-foo.txt`.
+. Add an entry for `git-foo` to the list at the end of
+ `Documentation/cmd-list.perl`.
+How a built-in is called
+The implementation `cmd_foo()` takes three parameters, `argc`, `argv,
+and `prefix`. The first two are similar to what `main()` of a
+standalone command would be called with.
+When `RUN_SETUP` is specified in the `commands[]` table, and when you
+were started from a subdirectory of the work tree, `cmd_foo()` is called
+after chdir(2) to the top of the work tree, and `prefix` gets the path
+to the subdirectory the command started from. This allows you to
+convert a user-supplied pathname (typically relative to that directory)
+to a pathname relative to the top of the work tree.
+The return value from `cmd_foo()` becomes the exit status of the