[gn build] use \bfoo\b instead of \<foo\> in sync script

\<foo\> is more correct, but since we use shell=True on Windows,
the < and > get interpreted as redirection operators.

Rather than adding cmd escaping, just use \bfoo\b, which is Good
Enough Often Enough.

llvm/utils/gn/build/sync_source_lists_from_cmake.py

@@ -70,7 +70,7 @@ def find_gitrev(touched_line, in_file):
         # undefined behavior according to the POSIX extended regex spec.
         posix_re_escape = lambda s: re.sub(r'([.[{()\\*+?|^$])', r'\\\1', s)
         cmd = ['log', '--format=%h', '-1', '--pickaxe-regex',
-               r'-S\<%s\>' % posix_re_escape(touched_line), in_file]
+               r'-S\b%s\b' % posix_re_escape(touched_line), in_file]
         return git_out(cmd).rstrip()
 
     # Collect changes to gn files, grouped by revision.