minimal/do: redo vs redo-ifchange, and fix empty target handling.

We previously assumed that redo and redo-ifchange are the same in
minimal/do's design, because it rebuilds all targets on every run, and
so there's no reason to ever build the same target more than once.

Unfortunately that's incorrect: if you run 'redo x' from two points in
a single run (or even twice in the same .do file), we expect x to be
built twice.  If you wanted redo to decide whether to build it the
second time, you should have used redo-ifchange.

t/102-empty/touchtest was trying to test for this.  However, a
second bug in minimal/do made the test pass anyway.  minimal/do would
*always* rebuild any target x that produced no output, not caring
whether it had tried to build before, whether you used redo or
redo-ifchange.  And while we tested that redo would redo a file that
had been deleted, we didn't ensure that it would redo a file that was
*not* deleted, nor that redo-ifchange would *not* redo that file.

Fix both bugs in minimal/do, and make t/102-empty/touchtest cover the
missing cases.

minimal/do

@@ -33,6 +33,7 @@ dirname()
 
 _dirsplit "$0"
 export REDO=$(cd "${dir:-.}" && echo "$PWD/$base")
+if [ "$base" = "redo-ifchange" ]; then ifchange=1; else ifchange=; fi
 
 DO_TOP=
 if [ -z "$DO_BUILT" ]; then
@@ -107,7 +108,7 @@ _run_dofile()
 _do()
 {
 	local dir="$1" target="$2" tmp="$3"
-	if [ ! -e "$target" ] || [ -d "$target" -a ! -e "$target.did" ]; then
+	if [ -z "$ifchange" ] || ( [ ! -e "$target" -o -d "$target" ] && [ ! -e "$target.did" ] ); then
 		printf '%sdo  %s%s%s%s\n' \
 			"$green" "$DO_DEPTH" "$bold" "$dir$target" "$plain" >&2
 		echo "$PWD/$target" >>"$DO_BUILT"

t/102-empty/clean.do

@@ -1 +1 @@
-rm -f touch1 *~ .*~
+rm -f touch1 touch1-ran *~ .*~

t/102-empty/touchtest.do

@@ -2,12 +2,40 @@
 # between "real" redo and minimal/do, so clean it up.
 rm -f touch1
 
+# simply create touch1
 echo 'echo hello' >touch1.do
 redo touch1
 [ -e touch1 ] || exit 55
+[ "$(cat touch1)" = "hello" ] || exit 56
+
+# ensure that 'redo touch1' always re-runs touch1.do even if we have
+# already built touch1 in this session, and even if touch1 already exists.
+echo 'echo hello2' >touch1.do
+redo touch1
+[ "$(cat touch1)" = "hello2" ] || exit 57
+
+# ensure that touch1 is rebuilt even if it got deleted after the last redo
+# inside the same session.  Also ensure that we can produce a zero-byte
+# output file explicitly.
 rm -f touch1
 echo 'touch $3' >touch1.do
 redo touch1
 [ -e touch1 ] || exit 66
-[ -z "$(cat touch1)" ] || exit 77
+[ -z "$(cat touch1)" ] || exit 67
+
+# Also test that zero bytes of output does not create the file at all, as
+# opposed to creating a zero-byte file.
+rm -f touch1
+echo 'touch touch1-ran' >touch1.do
+redo touch1
+[ -e touch1 ] && exit 75
+[ -e touch1-ran ] || exit 76
+rm -f touch1-ran
+
+# Make sure that redo-ifchange *won't* rebuild touch1 if we have already
+# built it, even if building it did not produce an output file.
+redo-ifchange touch1
+[ -e touch1 ] && exit 77
+[ -e touch1-ran ] && exit 78
+
 rm -f touch1.do