Change git credential from cache to store and also add test for

username/password case.

For cache to store change:
* By default, cache only last 900 seconds, gitsync will break after
  that. See https://git-scm.com/docs/git-credential-cache.
* The test won't work with cache since the test don't have access to
  the default unix socket location; XDG_CACHE_HOME override also can
  pre-create a socket in advance.
* `store` put the credential into a file, much easier to debug than cache.
* Considering anyone have access to the pod already able to get the
  credential via environment variables or yaml configs, so put it in
  file won't make it less secure.

For the new password test:
1. askpass_git.sh provided to simulate a git with password challenge.
2. Need and only need to similate "clone" action, need to bypass other
  actions like config/credential setup.
3. See `credential fill` is the official git action to ask password,
  see https://git-scm.com/docs/git-credential.

This change resolved issue #196.

askpass_git.sh

@@ -0,0 +1,25 @@
+#!/bin/sh
+# Ask pass when cloning new repo, fail if it mismatched the magic password.
+
+mkdir -p "${XDG_CONFIG_HOME}/git/"
+# Override the default 'git --global' config location, the default location
+# outside the e2e test environment. See https://git-scm.com/docs/git-config
+touch "${XDG_CONFIG_HOME}/git/config"
+# Override the default 'git credential store' config location, the default location
+# outside the e2e test environment. See https://git-scm.com/docs/git-credential-store
+touch "${XDG_CONFIG_HOME}/git/credentials"
+
+if [ "$1" != "clone" ]; then
+  git "$@"
+  exit $?
+fi
+
+REPO=$(echo "$@" | grep -o "file://[^ ]*")
+PASSWD=$(echo "url=${REPO}" | git credential fill | grep -o "password=.*")
+# Test case much match the magic password below.
+if [ "${PASSWD}" != "password=Lov3!k0os" ]; then
+  echo "invalid password ${PASSWD}, try Lov3!k0os next time."
+  exit 1
+fi
+
+git "$@"

cmd/git-sync/main.go

@@ -695,7 +695,7 @@ func runCommandWithStdin(ctx context.Context, cwd, stdin, command string, args .
 func setupGitAuth(ctx context.Context, username, password, gitURL string) error {
 	log.V(1).Info("setting up git credential cache")
 
-	_, err := runCommand(ctx, "", *flGitCmd, "config", "--global", "credential.helper", "cache")
+	_, err := runCommand(ctx, "", *flGitCmd, "config", "--global", "credential.helper", "store")
 	if err != nil {
 		return fmt.Errorf("error setting up git credentials: %v", err)
 	}

test_e2e.sh

@@ -89,6 +89,8 @@ function GIT_SYNC() {
         -u $(id -u):$(id -g) \
         -v "$DIR":"$DIR" \
         -v "$(pwd)/slow_git.sh":"/slow_git.sh" \
+        -v "$(pwd)/askpass_git.sh":"/askpass_git.sh" \
+        --env XDG_CONFIG_HOME=$DIR \
         --network="host" \
         --rm \
         e2e/git-sync:$(make -s version)__$(go env GOOS)_$(go env GOARCH) \
@@ -102,6 +104,7 @@ function remove_sync_container() {
 }
 
 SLOW_GIT=/slow_git.sh
+ASKPASS_GIT=/askpass_git.sh
 
 REPO="$DIR/repo"
 mkdir "$REPO"
@@ -621,6 +624,52 @@ remove_sync_container
 wait
 pass
 
+##############################################
+# Test password
+##############################################
+testcase "password"
+echo "$TESTCASE 1" > "$REPO"/file
+git -C "$REPO" commit -qam "$TESTCASE 1"
+# run with askpass_git but with wrong password
+GIT_SYNC \
+    --git=$ASKPASS_GIT \
+    --username="you@example.com" \
+    --password="I have no idea what the password is." \
+    --logtostderr \
+    --v=5 \
+    --wait=0.1 \
+    --repo="file://$REPO" \
+    --branch=master \
+    --rev=HEAD \
+    --root="$ROOT" \
+    --dest="link" \
+    > "$DIR"/log."$TESTCASE" 2>&1 &
+sleep 3
+# check for failure
+assert_file_absent "$ROOT"/link/file
+# run with askpass_git with correct password
+GIT_SYNC \
+    --git=$ASKPASS_GIT \
+    --username="you@example.com" \
+    --password="Lov3!k0os" \
+    --logtostderr \
+    --v=5 \
+    --wait=0.1 \
+    --repo="file://$REPO" \
+    --branch=master \
+    --rev=HEAD \
+    --root="$ROOT" \
+    --dest="link" \
+    > "$DIR"/log."$TESTCASE" 2>&1 &
+sleep 3
+assert_link_exists "$ROOT"/link
+assert_file_exists "$ROOT"/link/file
+assert_file_eq "$ROOT"/link/file "$TESTCASE 1"
+# Wrap up
+remove_sync_container
+wait
+pass
+
 ##############################################
 # Test webhook
 ##############################################
@@ -788,7 +837,7 @@ assert_file_absent "$ROOT"/link/$SUBMODULE_REPO_NAME/$NESTED_SUBMODULE_REPO_NAME
 # Remove submodule
 git -C "$REPO" submodule deinit -q $SUBMODULE_REPO_NAME
 rm -rf "$REPO"/.git/modules/$SUBMODULE_REPO_NAME
-git -C "$REPO" rm -qf $SUBMODULE_REPO_NAME 
+git -C "$REPO" rm -qf $SUBMODULE_REPO_NAME
 git -C "$REPO" commit -aqm "delete submodule"
 sleep 3
 assert_link_exists "$ROOT"/link