feat(gerrit-cli): Add gerrit CLI script

Add a script to run gerrit commands on the depot host, which reads the
gerrit username from the TVL_USERNAME env var or $(whoami) by default.

At some point this may want to solve the configuration problem in a more
general fashion, but for now this seems relatively obvious.

Change-Id: Ied91c1d26daf4770aef74b2e755d1760c486bb7b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/396
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: glittershark <grfn@gws.fyi>

bin/__dispatch.sh

@@ -50,6 +50,9 @@ case "${TARGET_TOOL}" in
   depot-build)
     attr="tools.depot-build"
     ;;
+  gerrit)
+    attr="tools.gerrit-cli"
+    ;;
   *)
     echo "The tool '${TARGET_TOOL}' is currently not installed in this repository."
     exit 1

bin/gerrit

@@ -0,0 +1 @@
+__dispatch.sh