cli/config: support user header index config

We don't do anything with this configuration information information
yet, but nonetheless add a couple of regression tests to make sure we
don't break standard functionality when we do use the configuration
information.

notmuch-config.c

@@ -802,6 +802,7 @@ typedef struct config_key {
 static struct config_key
 config_key_table[] = {
     {"index.decrypt",	true,	false,	NULL},
+    {"index.header.",	true,	true,	NULL},
     {"query.",		true,	true,	NULL},
 };
 

test/T750-user-header.sh

@@ -0,0 +1,43 @@
+#!/usr/bin/env bash
+test_description='indexing user specified headers'
+. $(dirname "$0")/test-lib.sh || exit 1
+
+test_begin_subtest "error adding user header before initializing DB"
+notmuch config set index.header.List List-Id 2>&1 | notmuch_dir_sanitize > OUTPUT
+cat <<EOF > EXPECTED
+Error opening database at MAIL_DIR/.notmuch: No such file or directory
+EOF
+test_expect_equal_file EXPECTED OUTPUT
+
+add_email_corpus
+
+notmuch search '*' | notmuch_search_sanitize > initial-threads
+notmuch search --output=messages '*' > initial-message-ids
+notmuch dump > initial-dump
+
+test_begin_subtest "adding user header"
+test_expect_code 0 "notmuch config set index.header.List \"List-Id\""
+
+test_begin_subtest "adding existing user header"
+test_expect_code 0 "notmuch config set index.header.List \"List-Id\""
+
+
+test_begin_subtest "retrieve user header"
+output=$(notmuch config get index.header.List)
+test_expect_equal "List-Id" "$output"
+
+test_begin_subtest 'reindex after adding header preserves threads'
+notmuch reindex '*'
+notmuch search '*' | notmuch_search_sanitize > OUTPUT
+test_expect_equal_file initial-threads OUTPUT
+
+test_begin_subtest "List all user headers"
+notmuch config set index.header.Spam "X-Spam"
+notmuch config list | grep ^index.header | notmuch_config_sanitize > OUTPUT
+cat <<EOF > EXPECTED
+index.header.List=List-Id
+index.header.Spam=X-Spam
+EOF
+test_expect_equal_file EXPECTED OUTPUT
+
+test_done