OS: Windows

1. Open Git Bash or other MSYS shell.
2. Run GN command that requires passing a path. Give an absolute path. For instance:
   gn refs out/Debug "/C:/foo/bar/*"

Expected result:
Specified path should be accepted as a valid GN-style absolute path.

Actual result:
GN reports an error:
  ERROR Bad absolute path.
  Absolute paths must be of the form /C:\ but this is "C:/foo/bar/*".

The root cause is that MSYS shells convert POSIX paths before passing them to Windows native programs. It means that the leading slash character is removed from path before it's passed to GN. See http://www.mingw.org/wiki/Posix_path_conversion