[PATCH 1/3] tests: add a special setup where for protocol.version

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Add a GIT_TEST_PROTOCOL_VERSION=X test mode which is equivalent to
running with protocol.version=X. This is needed to spot regressions
and differences such as "ls-refs" behaving differently with
transfer.hideRefs. See
https://public-inbox.org/git/20181211104236.GA6899@sigill.intra.peff.net/
for a fix for that regression.

With this all tests pass with GIT_TEST_PROTOCOL_VERSION=0, but fail
with GIT_TEST_PROTOCOL_VERSION=[1|2]. That's OK since this is a new
test mode, subsequent patches will fix up these test failures.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 protocol.c | 13 ++++++++++++-
 t/README   |  4 ++++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/protocol.c b/protocol.c
index 5e636785d1..cb58cbb29a 100644
--- a/protocol.c
+++ b/protocol.c
@@ -17,7 +17,18 @@ static enum protocol_version parse_protocol_version(const char *value)
 enum protocol_version get_protocol_version_config(void)
 {
 	const char *value;
-	if (!git_config_get_string_const("protocol.version", &value)) {
+	const char *git_test_k = "GIT_TEST_PROTOCOL_VERSION";
+	const char *git_test_v = getenv(git_test_k);
+
+	if (git_test_v) {
+		enum protocol_version version = parse_protocol_version(git_test_v);
+
+		if (version == protocol_unknown_version)
+			die("unknown value for %s: %s", git_test_k,
+			    git_test_v);
+
+		return version;
+	} else if (!git_config_get_string_const("protocol.version", &value)) {
 		enum protocol_version version = parse_protocol_version(value);
 
 		if (version == protocol_unknown_version)
diff --git a/t/README b/t/README
index 28711cc508..c5762a92bc 100644
--- a/t/README
+++ b/t/README
@@ -358,6 +358,10 @@ GIT_TEST_MULTI_PACK_INDEX=<boolean>, when true, forces the multi-pack-
 index to be written after every 'git repack' command, and overrides the
 'core.multiPackIndex' setting to true.
 
+GIT_TEST_PROTOCOL_VERSION=<'protocol.version' config value>, when set,
+runs the test suite with the given protocol.version. E.g. "0", "1" or
+"2".
+
 Naming Tests
 ------------
 
-- 
2.20.0.405.gbc1bbc6f85