Add user preference `HistoryBackwardSearchSkipIdenticalEntries` #5209

Job	Run time
Cross compiler for s390x	26m 44s
Test in OpenBSD	11m 48s
testinstall - - ubuntu-latest	4m 26s
teststandard - - ubuntu-latest	18m 53s
testmanuals - - ubuntu-latest	5m 47s
testexpect testmockpkg testspecial test-compile testlibgap testkernel testworkspace - - ubuntu-latest	6m 24s
testpackages testinstall-loadall - - ubuntu-latest	26m 1s
makemanuals testmakeinstall - CONFIGFLAGS="--prefix=/tmp/gapprefix" NO_COVERAGE=1 - ubuntu-latest	5m 27s
teststandard - ABI=32 CONFIGFLAGS="" - ubuntu-20.04	17m 59s
testinstall - HPCGAP=yes ABI=64 - ubuntu-latest	7m 25s
testmockpkg testinstall - BOOTSTRAP_MINIMAL=yes - macos-latest	7m 15s
testbugfix - CONFIGFLAGS="--enable-memory-checking" - ubuntu-latest	35m 23s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=64 BUILDDIR=out-of-tree CONFIGFLAGS="--enable-valgrind" - ubuntu-latest	3m 45s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=32 BUILDDIR=out-of-tree CONFIGFLAGS="" - ubuntu-20.04	2m 57s
testinstall - JULIA=yes CONFIGFLAGS="--enable-debug --disable-Werror" - ubuntu-20.04	4m 34s
testmockpkg testinstall - - windows-2019	15m 56s
Send Slack notification on status change	0s
	3h 20m 44s

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