builtin-am: implement skeletal builtin am

For the purpose of rewriting git-am.sh into a C builtin, implement a
skeletal builtin/am.c that redirects to $GIT_EXEC_PATH/git-am if the
environment variable _GIT_USE_BUILTIN_AM is not defined. Since in the
Makefile git-am.sh takes precedence over builtin/am.c,
$GIT_EXEC_PATH/git-am will contain the shell script git-am.sh, and thus
this allows us to fall back on the functional git-am.sh when running the
test suite for tests that depend on a working git-am implementation.

Since git-am.sh cannot handle any environment modifications by
setup_git_directory(), "am" is declared with no setup flags in git.c. On
the other hand, to re-implement git-am.sh in builtin/am.c, we need to
run all the git dir and work tree setup logic that git.c typically does
for us. As such, we work around this temporarily by copying the logic in
git.c's run_builtin(), which is roughly:

	prefix = setup_git_directory();
	trace_repo_setup(prefix);
	setup_work_tree();

This redirection should be removed when all the features of git-am.sh
have been re-implemented in builtin/am.c.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/builtin.h b/builtin.h
index 839483de6e..79aaf0afe8 100644
--- a/builtin.h
+++ b/builtin.h
@@ -30,6 +30,7 @@ extern int textconv_object(const char *path, unsigned mode, const unsigned char
 extern int is_builtin(const char *s);
 
 extern int cmd_add(int argc, const char **argv, const char *prefix);
+extern int cmd_am(int argc, const char **argv, const char *prefix);
 extern int cmd_annotate(int argc, const char **argv, const char *prefix);
 extern int cmd_apply(int argc, const char **argv, const char *prefix);
 extern int cmd_archive(int argc, const char **argv, const char *prefix);