test-lib: allow setting the index format version

Allow adding a TEST_GIT_INDEX_VERSION variable to config.mak to set the
index version with which the test suite should be run.

If it isn't set, the default version given in the source code is
used (currently version 3).

To avoid breakages with index versions other than [23], also set the
index version under which t2104 is run to 3.  This test only tests
functionality specific to version 2 and 3 of the index file and would
fail if the test suite is run with any other version.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -342,6 +342,10 @@ all::
 # Define DEFAULT_HELP_FORMAT to "man", "info" or "html"
 # (defaults to "man") if you want to have a different default when
 # "git help" is called without a parameter specifying the format.
+#
+# Define TEST_GIT_INDEX_VERSION to 2, 3 or 4 to run the test suite
+# with a different indexfile format version.  If it isn't set the index
+# file format used is index-v[23].
 
 GIT-VERSION-FILE: FORCE
 	@$(SHELL_PATH) ./GIT-VERSION-GEN
@@ -2222,6 +2226,9 @@ endif
 ifdef GIT_PERF_MAKE_OPTS
 	@echo GIT_PERF_MAKE_OPTS=\''$(subst ','\'',$(subst ','\'',$(GIT_PERF_MAKE_OPTS)))'\' >>$@
 endif
+ifdef TEST_GIT_INDEX_VERSION
+	@echo TEST_GIT_INDEX_VERSION=\''$(subst ','\'',$(subst ','\'',$(TEST_GIT_INDEX_VERSION)))'\' >>$@
+endif
 
 ### Detect Python interpreter path changes
 ifndef NO_PYTHON

t/t2104-update-index-skip-worktree.sh

@@ -7,6 +7,8 @@ test_description='skip-worktree bit test'
 
 . ./test-lib.sh
 
+test_set_index_version 3
+
 cat >expect.full <<EOF
 H 1
 H 2

t/test-lib-functions.sh

@@ -32,6 +32,11 @@ test_set_editor () {
 	export EDITOR
 }
 
+test_set_index_version () {
+    GIT_INDEX_VERSION="$1"
+    export GIT_INDEX_VERSION
+}
+
 test_decode_color () {
 	awk '
 		function name(n) {

t/test-lib.sh

@@ -108,6 +108,12 @@ export GIT_AUTHOR_EMAIL GIT_AUTHOR_NAME
 export GIT_COMMITTER_EMAIL GIT_COMMITTER_NAME
 export EDITOR
 
+if test -n "${TEST_GIT_INDEX_VERSION:+isset}"
+then
+	GIT_INDEX_VERSION="$TEST_GIT_INDEX_VERSION"
+	export GIT_INDEX_VERSION
+fi
+
 # Add libc MALLOC and MALLOC_PERTURB test
 # only if we are not executing the test with valgrind
 if expr " $GIT_TEST_OPTS " : ".* --valgrind " >/dev/null ||