Amend loads prev. msg. even if CWD is outside repo

The path to the ".git" directory returned by
git rev-parse --git-dir
and stored in "b:magit_git_dir", can be a relative one, which would not work for opening file
"COMMIT_EDITMSG" when the currently edited file was opened from outside the
repository, so we make "b:magit_git_dir" absolute via "fnamemodify".

autoload/magit/git.vim

@@ -84,7 +84,7 @@ function! magit#git#set_top_dir(path)
 			throw 'set_top_dir_error'
 		endtry
 		let b:magit_top_dir=top_dir
-		let b:magit_git_dir=git_dir
+		let b:magit_git_dir=fnamemodify(git_dir, ':p')
 	finally
 		call magit#utils#chdir(dir)
 	endtry