chore(scripts): check PR mergeable state before merging #252

Job	Run time
filter	1m 18s
build(8) except for gapic-generator-java	0s
build(25) except self-service clients	0s
dependency_analyzer_unit_test	0s
build	0s
showcase-clirr	0s
build(8) for gapic-generator-java	0s
unmanaged_dependency_check	0s
showcase	0s
build(8) for showcase	0s
build(21) except self-service clients	0s
gapic-generator-java-bom	0s
	1m 18s

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