Fix temp_files cache on Windows when TEMP is set to a short path (#893)

If %TEMP% is set to a short path (e.g. "C:\LongDi~1") then tempname()
will return a file name that contains that short path. If that path is
later used as key for entry in s:temp_files dictionary, that entry won't
be found in BufNewFile,BufReadPost events because <afile> is expand()'ed
before it is used as a key for s:temp_files.
In the end, user gets cryptic error message about
"C:\LongDirName\VI12345.tmp.fugitiveblame" not being a git repository
when he tries to open a commit in Gblame window.

To workaround that we expand paths of temp files when adding entries to
s:temp_files. Also, because expand() can't expand short path if it
doesn't exist in the file system, we have to extract the directory part
and expand it separately.

plugin/fugitive.vim

@@ -1390,6 +1390,9 @@ function! s:Edit(cmd,bang,...) abort
       return 'redraw|echo '.string(':!'.git.' '.args)
     else
       let temp = resolve(tempname())
+      if has('win32')
+        let temp = fnamemodify(fnamemodify(temp, ':h'), ':p').fnamemodify(temp, ':t')
+      endif
       let s:temp_files[s:cpath(temp)] = { 'dir': buffer.repo().dir(), 'args': arglist }
       silent execute a:cmd.' '.temp
       if a:cmd =~# 'pedit'
@@ -2011,6 +2014,9 @@ function! s:Blame(bang,line1,line2,count,args) abort
         endif
         let top = line('w0') + &scrolloff
         let current = line('.')
+        if has('win32')
+          let temp = fnamemodify(fnamemodify(temp, ':h'), ':p').fnamemodify(temp, ':t')
+        endif
         let s:temp_files[s:cpath(temp)] = { 'dir': s:repo().dir(), 'args': cmd }
         exe 'keepalt leftabove vsplit '.temp
         let b:fugitive_blamed_bufnr = bufnr