pretty: add abbrev option to %(describe)

The %(describe) placeholder by default, like `git describe`, uses a
seven-character abbreviated commit object name. This may not be
sufficient to fully describe all commits in a given repository,
resulting in a placeholder replacement changing its length because the
repository grew in size.  This could cause the output of git-archive to
change.

Add the --abbrev option to `git describe` to the placeholder interface
in order to provide tools to the user for fine-tuning project defaults
and ensure reproducible archives.

One alternative would be to just always specify --abbrev=40 but this may
be a bit too biased...

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -222,6 +222,10 @@ The placeholders are:
 +
 ** 'tags[=<bool>]': Instead of only considering annotated tags,
    consider lightweight tags as well.
+** 'abbrev=<number>': Instead of using the default number of hexadecimal digits
+   (which will vary according to the number of objects in the repository with a
+   default of 7) of the abbreviated object name, use <number> digits, or as many
+   digits as needed to form a unique object name.
 ** 'match=<pattern>': Only consider tags matching the given
    `glob(7)` pattern, excluding the "refs/tags/" prefix.
 ** 'exclude=<pattern>': Do not consider tags matching the given

pretty.c

@@ -1220,10 +1220,12 @@ static size_t parse_describe_args(const char *start, struct strvec *args)
 		char *name;
 		enum {
 			DESCRIBE_ARG_BOOL,
+			DESCRIBE_ARG_INTEGER,
 			DESCRIBE_ARG_STRING,
 		} type;
 	}  option[] = {
 		{ "tags", DESCRIBE_ARG_BOOL},
+		{ "abbrev", DESCRIBE_ARG_INTEGER },
 		{ "exclude", DESCRIBE_ARG_STRING },
 		{ "match", DESCRIBE_ARG_STRING },
 	};
@@ -1247,6 +1249,19 @@ static size_t parse_describe_args(const char *start, struct strvec *args)
 					found = 1;
 				}
 				break;
+			case DESCRIBE_ARG_INTEGER:
+				if (match_placeholder_arg_value(arg, option[i].name, &arg,
+								&argval, &arglen)) {
+					char *endptr;
+					if (!arglen)
+						return 0;
+					strtol(argval, &endptr, 10);
+					if (endptr - argval != arglen)
+						return 0;
+					strvec_pushf(args, "--%s=%.*s", option[i].name, (int)arglen, argval);
+					found = 1;
+				}
+				break;
 			case DESCRIBE_ARG_STRING:
 				if (match_placeholder_arg_value(arg, option[i].name, &arg,
 								&argval, &arglen)) {

t/t4205-log-pretty-formats.sh

@@ -1010,4 +1010,12 @@ test_expect_success '%(describe:tags) vs git describe --tags' '
 	test_cmp expect actual
 '
 
+test_expect_success '%(describe:abbrev=...) vs git describe --abbrev=...' '
+	test_when_finished "git tag -d tagname" &&
+	git tag -a -m tagged tagname &&
+	git describe --abbrev=15 >expect &&
+	git log -1 --format="%(describe:abbrev=15)" >actual &&
+	test_cmp expect actual
+'
+
 test_done