rev-parse: add a --show-object-format option

Add an option to print the object format used for input, output, or
storage. This allows shell scripts to discover the hash algorithm in
use.

Since the transition plan allows for multiple input algorithms, document
that we may provide multiple results for input, and the format that the
results may take. While we don't support this now, documenting it early
means that script authors can future-proof their scripts for when we do.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/builtin/rev-parse.c b/builtin/rev-parse.c
index f8bbe6d..6c5d5d6 100644
--- a/builtin/rev-parse.c
+++ b/builtin/rev-parse.c
@@ -918,6 +918,17 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 				show_datestring("--min-age=", arg);
 				continue;
 			}
+			if (opt_with_value(arg, "--show-object-format", &arg)) {
+				const char *val = arg ? arg : "storage";
+
+				if (strcmp(val, "storage") &&
+				    strcmp(val, "input") &&
+				    strcmp(val, "output"))
+					die("unknown mode for --show-object-format: %s",
+					    arg);
+				puts(the_hash_algo->name);
+				continue;
+			}
 			if (show_flag(arg) && verify)
 				die_no_single_rev(quiet);
 			continue;