Bump third_party/imgui/repo from `2860d7b` to `a6bcbb1` #1662

Job	Run time
Build on Linux (gcc_debug)	50m 37s
Build on Darwin (clang, simulated)	45m 47s
Build on Linux (fake, gcc_release, clang, simulated)	49m 28s
Build on Linux (coverage)	18m 19s
Build on Linux (python lighting-app)	4m 49s
Build on Linux (python_lib)	19m 8s
	3h 8m 8s

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