archive: provide builtin .tar.gz filter

This works exactly as if the user had configured it via:

  [tar "tgz"]
	command = gzip -cn
  [tar "tar.gz"]
	command = gzip -cn

but since it is so common, it's convenient to have it
builtin without the user needing to do anything.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-archive.txt

@@ -110,6 +110,9 @@ tar.<format>.command::
 	to the command (e.g., "-9"). An output file with the same
 	extension as `<format>` will be use this format if no other
 	format is given.
++
+The "tar.gz" and "tgz" formats are defined automatically and default to
+`gzip -cn`. You may override them with custom commands.
 
 ATTRIBUTES
 ----------
@@ -143,6 +146,14 @@ git archive --format=tar --prefix=git-1.4.0/ v1.4.0 | gzip >git-1.4.0.tar.gz::
 
 	Create a compressed tarball for v1.4.0 release.
 
+git archive --format=tar.gz --prefix=git-1.4.0/ v1.4.0 >git-1.4.0.tar.gz::
+
+	Same as above, but using the builtin tar.gz handling.
+
+git archive --prefix=git-1.4.0/ -o git-1.4.0.tar.gz v1.4.0::
+
+	Same as above, but the format is inferred from the output file.
+
 git archive --format=tar --prefix=git-1.4.0/ v1.4.0{caret}\{tree\} | gzip >git-1.4.0.tar.gz::
 
 	Create a compressed tarball for v1.4.0 release, but without a

archive-tar.c

@@ -357,6 +357,8 @@ void init_tar_archiver(void)
 	int i;
 	register_archiver(&tar_archiver);
 
+	tar_filter_config("tar.tgz.command", "gzip -cn", NULL);
+	tar_filter_config("tar.tar.gz.command", "gzip -cn", NULL);
 	git_config(git_tar_config, NULL);
 	for (i = 0; i < nr_tar_filters; i++) {
 		/* omit any filters that never had a command configured */

t/t5000-tar-tree.sh

@@ -26,6 +26,8 @@ commit id embedding:
 
 . ./test-lib.sh
 UNZIP=${UNZIP:-unzip}
+GZIP=${GZIP:-gzip}
+GUNZIP=${GUNZIP:-gzip -d}
 
 SUBSTFORMAT=%H%n
 
@@ -295,4 +297,40 @@ test_expect_success 'extension matching requires dot' '
 	test_cmp b.tar config-implicittar.foo
 '
 
+if $GZIP --version >/dev/null 2>&1; then
+	test_set_prereq GZIP
+else
+	say "Skipping some tar.gz tests because gzip not found"
+fi
+
+test_expect_success GZIP 'git archive --format=tgz' '
+	git archive --format=tgz HEAD >j.tgz
+'
+
+test_expect_success GZIP 'git archive --format=tar.gz' '
+	git archive --format=tar.gz HEAD >j1.tar.gz &&
+	test_cmp j.tgz j1.tar.gz
+'
+
+test_expect_success GZIP 'infer tgz from .tgz filename' '
+	git archive --output=j2.tgz HEAD &&
+	test_cmp j.tgz j2.tgz
+'
+
+test_expect_success GZIP 'infer tgz from .tar.gz filename' '
+	git archive --output=j3.tar.gz HEAD &&
+	test_cmp j.tgz j3.tar.gz
+'
+
+if $GUNZIP --version >/dev/null 2>&1; then
+	test_set_prereq GUNZIP
+else
+	say "Skipping some tar.gz tests because gunzip was not found"
+fi
+
+test_expect_success GZIP,GUNZIP 'extract tgz file' '
+	$GUNZIP -c <j.tgz >j.tar &&
+	test_cmp b.tar j.tar
+'
+
 test_done