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 "git rev-parse --absolute-git-dir".

Thanks to user v-slava for suggesting the use of --absolute-git-dir.

autoload/magit/git.vim

@@ -79,7 +79,7 @@ function! magit#git#set_top_dir(path)
 		try
 			let top_dir=magit#utils#strip(
 						\ system(g:magit_git_cmd . " rev-parse --show-toplevel")) . "/"
-			let git_dir=magit#utils#strip(system(g:magit_git_cmd . " rev-parse --git-dir")) . "/"
+			let git_dir=magit#utils#strip(system(g:magit_git_cmd . " rev-parse --absolute-git-dir")) . "/"
 			if ( executable("cygpath") )
 				let top_dir = magit#utils#strip(system("cygpath " . top_dir))
 				let git_dir = magit#utils#strip(system("cygpath " . git_dir))