build: Use git rev-parse to accomodate older gits (argoproj#3497)

Makefile

@@ -6,7 +6,7 @@ OUTPUT_IMAGE_ARCH ?= amd64
 BUILD_DATE             = $(shell date -u +'%Y-%m-%dT%H:%M:%SZ')
 GIT_COMMIT             = $(shell git rev-parse HEAD)
 GIT_REMOTE             = origin
-GIT_BRANCH             = $(shell git branch --show-current)
+GIT_BRANCH             = $(shell git rev-parse --symbolic-full-name --verify --quiet --abbrev-ref HEAD)
 GIT_TAG                = $(shell git describe --exact-match --tags HEAD 2>/dev/null || git rev-parse --short=8 HEAD 2>/dev/null)
 GIT_TREE_STATE         = $(shell if [ -z "`git status --porcelain`" ]; then echo "clean" ; else echo "dirty"; fi)