Bump third_party/imgui/repo from `2860d7b` to `39585aa` #1482

Job	Run time
REPL Tests - Linux	5m 34s
REPL Tests - Darwin	0s
Test Suites - Linux (no-ble-no-shell-tsan-clang)	5m 41s
Test Suites - Darwin (no-ble-no-shell-asan-clang)	41s
Test Suites - Darwin (no-ble-no-shell-tsan-clang)	7s
	12m 3s

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