add variant sample

variant_path/.gitignore

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+/build-default
+!/build-default/tup.config

variant_path/Tuprules.tup

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+&templates_dir = ./templates
+
+# This doesn't work, cannot find the generated header.txt
+# I want to access the variant path for templates/ but $(TUP_VARIANTDIR) will
+# follow the current Tupfile and there is no variant "anchor" like &-variables.
+!add_header = | &(templates_dir)/<header_group> |> cat &(templates_dir)/header.txt %f &(templates_dir)/footer.txt > %o |> %B.gen.txt
+
+# This works
+#!add_header = | &(templates_dir)/<header_group> |> cat %f &(templates_dir)/footer.txt > %o |> %B.gen.txt

variant_path/nested/a/Tupfile

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+include_rules
+
+: a.txt |> !add_header |>

variant_path/nested/a/a.txt

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+letter a

variant_path/nested/a/b/Tupfile

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+include_rules
+
+: b.txt |> !add_header |>

variant_path/nested/a/b/b.txt

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+letter b

variant_path/nested/a/b/c/Tupfile

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+include_rules
+
+: c.txt |> !add_header |>

variant_path/nested/a/b/c/c.txt

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+letter c

variant_path/templates/Tupfile

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+: |> ./generate_header.sh > %o |> header.txt ./<header_group>

variant_path/templates/footer.txt

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+-- 
+the end

variant_path/templates/generate_header.sh

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+#!/usr/bin/env bash
+
+set -euo pipefail
+
+cat << EOF
+YEAR $(date +%Y)
+====
+EOF