perf: add function to setup a fresh test repo

Add a function to setup a fresh test repo via 'git init' to compliment
the existing functions to copy over a normal & large repo.

Some performance tests don't need any existing repository data at all
to be significant, e.g. tests which stress glob matches against single
pathological revisions or files, which I'm about to add in a
subsequent commit.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/perf/README

@@ -106,6 +106,7 @@ sources perf-lib.sh:
 
 After that you will want to use some of the following:
 
+	test_perf_fresh_repo    # sets up an empty repository
 	test_perf_default_repo  # sets up a "normal" repository
 	test_perf_large_repo    # sets up a "large" repository
 

t/perf/perf-lib.sh

@@ -78,6 +78,10 @@ if test -z "$GIT_PERF_LARGE_REPO"; then
 	GIT_PERF_LARGE_REPO=$TEST_DIRECTORY/..
 fi
 
+test_perf_do_repo_symlink_config_ () {
+	test_have_prereq SYMLINKS || git config core.symlinks false
+}
+
 test_perf_create_repo_from () {
 	test "$#" = 2 ||
 	error "bug in the test script: not 2 parameters to test-create-repo"
@@ -102,15 +106,22 @@ test_perf_create_repo_from () {
 	) &&
 	(
 		cd "$repo" &&
-		"$MODERN_GIT" init -q && {
-			test_have_prereq SYMLINKS ||
-			git config core.symlinks false
-		} &&
+		"$MODERN_GIT" init -q &&
+		test_perf_do_repo_symlink_config_ &&
 		mv .git/hooks .git/hooks-disabled 2>/dev/null
 	) || error "failed to copy repository '$source' to '$repo'"
 }
 
 # call at least one of these to establish an appropriately-sized repository
+test_perf_fresh_repo () {
+	repo="${1:-$TRASH_DIRECTORY}"
+	"$MODERN_GIT" init -q "$repo" &&
+	(
+		cd "$repo" &&
+		test_perf_do_repo_symlink_config_
+	)
+}
+
 test_perf_default_repo () {
 	test_perf_create_repo_from "${1:-$TRASH_DIRECTORY}" "$GIT_PERF_REPO"
 }