Fix Gblame problems on Windows

On Windows, if Vim is invoked with a working directory starting with a lowercase drive letter (e.g. `c:\<path>` instead of `C:\<path>`), some mappings in the blame buffer do not work correctly.  For example, hitting Enter on a line throws an exception rather than showing the associated commit.  The reason for this is that the `b:git_dir` variable is not being set on the blame buffer.  The reason in turn for this is that the path to the blame buffer is being stored in `s:temp_files` with an uppercase drive letter, but in the `fugitive_temp` augroup, `'<afile>:p'` is being expanded with a lowercase drive letter, so the lookup in `s:temp_files` fails.  Fix this by converting any lowercase drive letter to uppercase before using a path as a key for `s:tempfiles`.