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 paths to lowercase before
using them as keys for the s:tempfile dictionary.  Because of the way
Vim generates temporary file names, this is safe even on platforms with
case-sensitive file systems.