gn build: Fix a Python2ism in write_vcsrevision.py.

[pcc]

Summary

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'"

[phosek]

LGTM

llvm/trunk/utils/gn/build/write_vcsrevision.py

def main():
 
     git, use_shell = which('git'), False
     if not git:
         git = which('git.exe')
     if not git:
         git = which('git.bat')
         use_shell = True
     rev = subprocess.check_output([git, 'rev-parse', '--short', 'HEAD'],
-                                  cwd=git_dir, shell=use_shell)
+                                  cwd=git_dir, shell=use_shell).decode().strip()
     # FIXME: add pizzas such as the svn revision read off a git note?
-    vcsrevision_contents = '#define LLVM_REVISION "git-%s"\n' % rev.strip()
+    vcsrevision_contents = '#define LLVM_REVISION "git-%s"\n' % rev
 
     # If the output already exists and is identical to what we'd write,
     # return to not perturb the existing file's timestamp.
     if os.path.exists(args.vcs_header) and \
             open(args.vcs_header).read() == vcsrevision_contents:
         return 0
 
     # http://neugierig.org/software/blog/2014/11/binary-revisions.html