[Outreachy PATCH v2 1/2] fast-import: handle --git-completion-helper before reading input

[Tanga Joris <tangajoris@gmail.com>]

fast-import parses its options only after reading the "feature"
lines from its input, and it creates a temporary pack file before
that. So "git fast-import --git-completion-helper" waits for input,
and each run leaves a tmp_pack_* file behind.

bash completion runs the helper with the terminal as input, so
"git fast-import --<TAB>" would freeze once fast-import is no
longer marked with the NO_PARSEOPT flag.

Handle a lone --git-completion-helper or --git-completion-helper-all
right after the -h check, before creating the pack or reading input.

Reported-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Tanga Joris <tangajoris@gmail.com>
---
Changes since v1:
 - New patch 1/2, addressing René's observation that completion would
   block on fast-import
 - 2/2 is unchanged

 builtin/fast-import.c  | 4 ++++
 t/t9300-fast-import.sh | 8 ++++++++
 2 files changed, 12 insertions(+)

diff --git a/builtin/fast-import.c b/builtin/fast-import.c
index 9db3f83eea..f53113d5a5 100644
--- a/builtin/fast-import.c
+++ b/builtin/fast-import.c
@@ -4204,6 +4204,10 @@ int cmd_fast_import(int argc,
 	};
 
 	show_usage_with_options_if_asked(argc, argv, fast_import_usage, fast_import_options);
+	if (argc == 2 && (!strcmp(argv[1], "--git-completion-helper") ||
+			  !strcmp(argv[1], "--git-completion-helper-all")))
+		parse_options(argc, argv, prefix, fast_import_options,
+			      fast_import_usage, 0);
 
 	fast_import_state_init(&state, argc, argv, prefix, fast_import_options);
 
diff --git a/t/t9300-fast-import.sh b/t/t9300-fast-import.sh
index 73c6e3d448..99d7843217 100755
--- a/t/t9300-fast-import.sh
+++ b/t/t9300-fast-import.sh
@@ -2914,6 +2914,14 @@ test_expect_success 'R: blob appears only once' '
 	test 1 = $n
 '
 
+test_expect_success 'R: --git-completion-helper does not read from stdin' '
+	find .git/objects/pack -name "tmp_pack_*" >before &&
+	echo garbage | git fast-import --git-completion-helper >out &&
+	test_grep -e "--date-format=" out &&
+	find .git/objects/pack -name "tmp_pack_*" >after &&
+	test_cmp before after
+'
+
 ###
 ### series S (mark and path parsing)
 ###
-- 
2.56.0.291.g0b48760bf4