Make the git-llvm script work on older git versions that don't support git rev-parse...

Make the git-llvm script work on older git versions that don't support git rev-parse --git-common-dir.

Not all versions of git support git rev-parse --git-common-dir. Rather than erorr or print any kind of
useful error, they just print back '--git-common-dir' instead of a directory. The git-llvm script
ends up taking this '--git-common-dir' as a diretory name to use.

Not sure exactly what happens after that, but the end result is that the 'git llvm push' ends up
looking like it pushed your commits, but really did nothing.

This patch makes the script detect the bogus directory name for --git-common-dir and falls back to using --git-dir instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@359939 91177308-0d34-0410-b5e6-96231b3b80d8