fix: don't create new fishfile (except the first time); fix #460

fisher.fish

@@ -159,12 +159,8 @@ function _fisher_commit
     if test ! -e "$fishfile"
         command touch $fishfile
         echo "created empty fishfile in $fishfile" | command sed "s|$HOME|~|" >&2
-    else if test -L "$fishfile"
-        set fishfile (command readlink $fishfile)
     end
-    _fisher_fishfile_indent (echo -s $argv\;) < $fishfile > $fishfile@
-    command mv -f $fishfile@ $fishfile
-    command rm -f $fishfile@
+    printf "%s\n" (_fisher_fishfile_indent (echo -s $argv\;) < $fishfile) > $fishfile
 
     set -l expected_pkgs (_fisher_fishfile_load < $fishfile)
     set -l added_pkgs (_fisher_pkg_fetch_all $expected_pkgs)