init: disallow --separate-git-dir with bare repository

The purpose of "git init --separate-git-dir" is to separate the
repository from the worktree. This is true even when --separate-git-dir
is used on an existing worktree, in which case, it moves the .git/
subdirectory to a new location outside the worktree.

However, an outright bare repository (such as one created by "git init
--bare"), has no worktree, so using --separate-git-dir to separate it
from its non-existent worktree is nonsensical. Therefore, make it an
error to use --separate-git-dir on a bare repository.

Implementation note: "git init" considers a repository bare if told so
explicitly via --bare or if it guesses it to be so based upon
heuristics. In the explicit --bare case, a conflict with
--separate-git-dir is easy to detect early. In the guessed case,
however, the conflict can only be detected once "bareness" is guessed,
which happens after "git init" has begun creating the repository.
Technically, we can get by with a single late check which would cover
both cases, however, erroring out early, when possible, without leaving
detritus provides a better user experience.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/init-db.c

@@ -568,6 +568,9 @@ int cmd_init_db(int argc, const char **argv, const char *prefix)
 
 	argc = parse_options(argc, argv, prefix, init_db_options, init_db_usage, 0);
 
+	if (real_git_dir && is_bare_repository_cfg == 1)
+		die(_("--separate-git-dir and --bare are mutually exclusive"));
+
 	if (real_git_dir && !is_absolute_path(real_git_dir))
 		real_git_dir = real_pathdup(real_git_dir, 1);
 
@@ -663,6 +666,8 @@ int cmd_init_db(int argc, const char **argv, const char *prefix)
 				   get_git_work_tree());
 	}
 	else {
+		if (real_git_dir)
+			die(_("--separate-git-dir incompatible with bare repository"));
 		if (work_tree)
 			set_git_work_tree(work_tree);
 	}

t/t0001-init.sh

@@ -316,6 +316,19 @@ test_expect_success 'init with separate gitdir' '
 	test_path_is_dir realgitdir/refs
 '
 
+test_expect_success 'explicit bare & --separate-git-dir incompatible' '
+	test_must_fail git init --bare --separate-git-dir goop.git bare.git 2>err &&
+	test_i18ngrep "mutually exclusive" err
+'
+
+test_expect_success 'implicit bare & --separate-git-dir incompatible' '
+	test_when_finished "rm -rf bare.git" &&
+	mkdir -p bare.git &&
+	test_must_fail env GIT_DIR=. \
+		git -C bare.git init --separate-git-dir goop.git 2>err &&
+	test_i18ngrep "incompatible" err
+'
+
 test_lazy_prereq GETCWD_IGNORES_PERMS '
 	base=GETCWD_TEST_BASE_DIR &&
 	mkdir -p $base/dir &&