We sometimes missed deps when more than one dep required a stamp check.

If must_build was nonempty when recursively calling isdirty() that
returned a list, we'd lose the original value of must_build.

t/660-stamp/.gitignore

@@ -4,3 +4,9 @@
 /inp
 /bob
 /usestamp2
+/a
+/b
+/ab
+/doing_ab
+/abc
+/doing_abc

t/660-stamp/a.do

@@ -0,0 +1,3 @@
+redo-always
+echo a | redo-stamp
+echo a > $3

t/660-stamp/ab.do

@@ -0,0 +1,4 @@
+redo-ifchange a b
+echo "doing ab" >&2
+echo ab >$3
+touch doing_ab

t/660-stamp/abc.do

@@ -0,0 +1,4 @@
+redo-ifchange ab c
+echo "doing abc" >&2
+echo abc >$3
+touch doing_abc

t/660-stamp/b.do

@@ -0,0 +1,3 @@
+redo-always
+echo b | redo-stamp
+echo b > $3

t/660-stamp/clean.do

@@ -1 +1 @@
-rm -f *.log usestamp usestamp2 stampy inp bob *~ .*~
+rm -f *.log usestamp usestamp2 stampy inp bob *~ .*~ a b ab doing_ab

t/660-stamp/stamptest.do

@@ -70,3 +70,16 @@ redo-ifchange usestamp usestamp2
 rm -f stampy
 redo-ifchange stampy
 [ "$(wc -l <stampy.log)" -eq 6 ] || exit 74
+
+# check that a target that depends on two stamped targets (that are marked as
+# always built) won't be redone if neither stamped targets changes
+../flush-cache
+redo-ifchange abc
+rm -f doing_ab doing_abc
+redo-ifchange abc
+[ ! -f doing_ab ] || exit 92
+[ ! -f doing_abc ] || exit 93
+../flush-cache
+redo-ifchange abc
+[ ! -f doing_ab ] || exit 94
+[ ! -f doing_abc ] || exit 95