bisect: do not run show-branch just to show the current commit

In scripted versions of "git bisect", we used "git show-branch" to
describe a single commit in the bisect log and also to the interactive
user after checking out the next version to be tested.

The former use of "git show-branch" was lost when the helper
function that wrote bisect log entries was rewritten at 0f30233
(bisect--helper: `bisect_write` shell function in C, 2019-01-02) in
C

But we've kept the latter ever since 0871984 (bisect: make "git
bisect" use new "--next-all" bisect-helper function, 2009-05-09)
started using the faithful C-rewrite introduced at ef24c7c
(bisect--helper: add "--next-exit" to output bisect results,
2009-04-19).

Showing "[<full hex>] <subject>" is simple enough with our helper
pretty.c::format_commit_message() and spawning show-branch is an
overkill.  Let's lose one external process.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: gitster

bisect.c

@@ -23,7 +23,6 @@ static struct oid_array skipped_revs;
 static struct object_id *current_bad_oid;
 
 static const char *argv_checkout[] = {"checkout", "-q", NULL, "--", NULL};
-static const char *argv_show_branch[] = {"show-branch", NULL, NULL};
 
 static const char *term_bad;
 static const char *term_good;
@@ -729,6 +728,9 @@ static enum bisect_error bisect_checkout(const struct object_id *bisect_rev, int
 {
 	char bisect_rev_hex[GIT_MAX_HEXSZ + 1];
 	enum bisect_error res = BISECT_OK;
+	struct commit *commit;
+	struct pretty_print_context pp = {0};
+	struct strbuf commit_msg = STRBUF_INIT;
 
 	oid_to_hex_r(bisect_rev_hex, bisect_rev);
 	update_ref(NULL, "BISECT_EXPECTED_REV", bisect_rev, NULL, 0, UPDATE_REFS_DIE_ON_ERR);
@@ -748,13 +750,11 @@ static enum bisect_error bisect_checkout(const struct object_id *bisect_rev, int
 			return -abs(res);
 	}
 
-	argv_show_branch[1] = bisect_rev_hex;
-	res = run_command_v_opt(argv_show_branch, RUN_GIT_CMD);
-	/*
-	 * Errors in `run_command()` itself, signaled by res < 0,
-	 * and errors in the child process, signaled by res > 0
-	 * can both be treated as regular BISECT_FAILURE (-1).
-	 */
+	commit = lookup_commit_reference(the_repository, bisect_rev);
+	format_commit_message(commit, "[%H] %s%n", &commit_msg, &pp);
+	fputs(commit_msg.buf, stdout);
+	strbuf_release(&commit_msg);
+
 	return -abs(res);
 }