Use version defined in Makefile in rocksdb_build_git_sha

Summary: Now rocksdb_build_git_sha is determined from "git sha". It is hard if the release is not from the repository directly but from a source code copy. Change to use the versions given in Makefile.

Test Plan: Run "make util/build_version.cc"

Reviewers: kradhakrishnan, rven, meyering, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D38451

Makefile

@@ -177,7 +177,7 @@ CXXFLAGS += $(WARNING_FLAGS) -I. -I./include $(PLATFORM_CXXFLAGS) $(OPT) -Woverl
 LDFLAGS += $(PLATFORM_LDFLAGS)
 
 date := $(shell date +%F)
-git_sha := $(shell git describe HEAD 2>/dev/null)
+git_sha := $(shell git rev-parse HEAD 2>/dev/null)
 gen_build_version =							\
   printf '%s\n'								\
     '\#include "build_version.h"'					\