fix(ping/rust): Use refs/heads/master and not branch and master

Follows suggestion in
libp2p#79 (comment).

Follow-up to libp2p#79 and
libp2p#80.

Author: mxinden

ping/_compositions/rust-cross-versions.toml

@@ -25,7 +25,7 @@
 
     [groups.build_config.build_args]
       BINARY_NAME = '{{ .BinaryName }}'
-      GIT_REF = "master"
+      GIT_REF = "refs/heads/master"
   {{ end }}
 
   {{ if $.Env.GitReference }}

ping/rust/Dockerfile

@@ -16,7 +16,7 @@ ARG GIT_TARGET=""
 RUN if [ ! -z "${GIT_TARGET}" ]; then sed -i "s,^git.*,git = \"https://${GIT_TARGET}\"," ./plan/Cargo.toml; fi
 
 ARG GIT_REF=""
-RUN if [ "master" = "${GIT_REF}" ]; then sed -i "s/^rev.*/branch= \"master\"/" ./plan/Cargo.toml; elif [ ! -z "${GIT_REF}" ]; then sed -i "s/^rev.*/rev = \"${GIT_REF}\"/" ./plan/Cargo.toml; fi
+RUN if [ ! -z "${GIT_REF}" ]; then sed -i "s/^rev.*/rev = \"${GIT_REF}\"/" ./plan/Cargo.toml; fi
 
 COPY ./plan/${PLAN_PATH}/src/lib.rs ./plan/src/lib.rs
 COPY ./plan/${PLAN_PATH}/src/bin/ ./plan/src/bin/