minimal/do: don't create a .did file until after a file is actually built.

With the new "continue" feature on by default, it turned out that
ctrl-c during a build, or a .do file returning an error, would mark a
target as "built" even though it hadn't been.  This would prevent
retrying it when you started minimal/do again.  Use a temp file
instead.

It's a little tricky: to prevent accidental recursion, we want to
create a file *before* building, but clean up that file when starting
the next session.  And we rename that file to the actual .did file
*after* building successfully.

t/360-symlinks/b.do

@@ -1,3 +1,3 @@
-echo x >>b.did
+echo x >>b.ran
 redo-ifchange a
 cat a >$3