Ensure we get the commit hash in FBINK_VERSION on shallow clones

(e.g., CI. I don't care enough about it to force unshallow clones in
there).

Makefile

@@ -302,7 +302,7 @@ EXTRA_CPPFLAGS+=$(TARGET_CPPFLAGS)
 
 # A version tag...
 # NOTE: Don't redirect stderr, so we can get an idea of what happened when things go wrong...
-FBINK_VERSION:=$(shell git describe || cat VERSION)
+FBINK_VERSION:=$(shell git describe || git rev-parse --short HEAD || cat VERSION)
 # Only use it if we got something useful...
 ifdef FBINK_VERSION
 	LIB_CFLAGS+=-DFBINK_VERSION='"$(FBINK_VERSION)"'