[PATCH 2/4] builtin/rev-parse: learn --null-oid

[Denton Liu <liu.denton@gmail.com>]

When a user needed the null OID for scripting purposes, it used to be
very easy: hardcode 40 zeros. However, since Git started supporting
SHA-256, this assumption became false which may break some scripts.
Allow users to fix their broken scripts by providing users with a
hash-agnostic method of obtaining the null OID.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
---
 Documentation/git-rev-parse.txt | 4 ++++
 builtin/rev-parse.c             | 4 ++++
 t/t1500-rev-parse.sh            | 6 ++++++
 3 files changed, 14 insertions(+)

diff --git a/Documentation/git-rev-parse.txt b/Documentation/git-rev-parse.txt
index 19b12b6d43..b370d425d7 100644
--- a/Documentation/git-rev-parse.txt
+++ b/Documentation/git-rev-parse.txt
@@ -285,6 +285,10 @@ print a message to stderr and exit with nonzero status.
 Other Options
 ~~~~~~~~~~~~~
 
+--null-oid::
+	Print the null OID (the OID containing all zeros). This OID is
+	used to represent a non-existent object.
+
 --since=datestring::
 --after=datestring::
 	Parse the date string, and output the corresponding
diff --git a/builtin/rev-parse.c b/builtin/rev-parse.c
index ed200c8af1..4e4ca99775 100644
--- a/builtin/rev-parse.c
+++ b/builtin/rev-parse.c
@@ -910,6 +910,10 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 				}
 				continue;
 			}
+			if (!strcmp(arg, "--null-oid")) {
+				puts(oid_to_hex(&null_oid));
+				continue;
+			}
 			if (skip_prefix(arg, "--since=", &arg)) {
 				show_datestring("--max-age=", arg);
 				continue;
diff --git a/t/t1500-rev-parse.sh b/t/t1500-rev-parse.sh
index 408b97d5af..8c1bd543ef 100755
--- a/t/t1500-rev-parse.sh
+++ b/t/t1500-rev-parse.sh
@@ -185,4 +185,10 @@ test_expect_success 'showing the superproject correctly' '
 	test_cmp expect out
 '
 
+test_expect_success 'rev-parse --null-oid' '
+	echo "$(test_oid zero)" >expect &&
+	git rev-parse --null-oid >actual &&
+	test_cmp expect actual
+'
+
 test_done
-- 
2.28.0.618.gf4bc123cb7