gn build: Fix a Python2ism in write_vcsrevision.py.

Convert the output of "git rev-parse --short HEAD" to a string before
substituting it into the output file. Without this the output file
will look like this on Python 3:

 #define LLVM_REVISION "git-b'6a4895a025f'"

Differential Revision: https://reviews.llvm.org/D56459

llvm-svn: 350686