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/201-fail/all.do

@@ -4,7 +4,6 @@ rm -f this-doesnt-exist
 redo-ifcreate this-doesnt-exist >/dev/null 2>&1 || exit 34  # expected to pass
 
 
-
 rm -f fail
 ! redo-ifchange fail >/dev/null 2>&1 || exit 44  # expected to fail
 
@@ -16,3 +15,14 @@ redo-ifchange fail >/dev/null 2>&1 || exit 55  # expected to pass
 # Make sure we don't leave this lying around for future runs, or redo
 # might mark it as "manually modified" (since we did!)
 rm -f fail
+
+rm -f maybe-fail
+: >want-fail
+! redo-ifchange maybe-fail >/dev/null 2>&1 || exit 66
+rm -f want-fail
+../flush-cache
+redo-ifchange maybe-fail || exit 67  # failed last time, must retry
+: >want-fail
+../flush-cache
+redo-ifchange maybe-fail || exit 68  # passed last time, no dep, no redo
+rm -f want-fail