Half-support for using file checksums instead of stamps.

A new redo-stamp program takes whatever you give it as stdin and uses it to
calculate a checksum for the current target.  If that checksum is the same
as last time, then we consider the target to be unchanged, and we set
checked_runid and stamp, but leave changed_runid alone.  That will make
future callers of redo-ifchange see this target as unmodified.

However, this is only "half" support because by the time we run the .do
script that calls redo-stamp, it's too late; the caller is a dependant of
the stamped program, which is already being rebuilt, even if redo-stamp
turns out to say that this target is unchanged.

The other half is coming up.

t/stamp/.gitignore

@@ -0,0 +1,4 @@
+/*.log
+/usestamp
+/stampy
+/inp

t/stamp/clean.do

@@ -0,0 +1 @@
+rm -f *.log usestamp stampy inp *~ .*~

t/stamp/stamptest.do

@@ -0,0 +1,29 @@
+rm -f stampy usestamp stampy.log usestamp.log
+echo one >inp
+
+../flush-cache.sh
+redo stampy
+[ "$(wc -l <stampy.log)" -eq 1 ] || exit 11
+
+redo-ifchange usestamp
+[ "$(wc -l <stampy.log)" -eq 1 ] || exit 21
+[ "$(wc -l <usestamp.log)" -eq 1 ] || exit 12
+
+../flush-cache.sh
+redo stampy
+[ "$(wc -l <stampy.log)" -eq 2 ] || exit 31
+[ "$(wc -l <usestamp.log)" -eq 1 ] || exit 32
+
+redo-ifchange usestamp
+[ "$(wc -l <stampy.log)" -eq 2 ] || exit 41
+[ "$(wc -l <usestamp.log)" -eq 1 ] || exit 42
+
+../flush-cache.sh
+echo two >inp
+redo stampy
+[ "$(wc -l <stampy.log)" -eq 3 ] || exit 51
+[ "$(wc -l <usestamp.log)" -eq 1 ] || exit 52
+
+redo-ifchange usestamp
+[ "$(wc -l <stampy.log)" -eq 3 ] || exit 61
+[ "$(wc -l <usestamp.log)" -eq 2 ] || exit 62

t/stamp/stampy.do

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+echo $$ >>stampy.log
+cat inp
+redo-stamp <inp

t/stamp/test.do

@@ -0,0 +1 @@
+redo stamptest

t/stamp/usestamp.do

@@ -0,0 +1,3 @@
+redo-ifchange stampy
+echo $$ >>usestamp.log
+cat stampy