Allow mercury footer/header update based on content #2637

Job	Run time
Emulator (universal, debuglink, x86_64, noasan)	3m 27s
Emulator arm	0s
Emulator (btconly, debuglink, x86_64, noasan)	3m 40s
Firmware (universal, debuglink)	3m 57s
Post comment with UI diff URLs	18s
Firmware (btconly, debuglink)	3m 50s
Upgrade test (noasan)	4m 9s
Device test (universal, noasan)	9m 5s
Upload emulator binaries	0s
Device test (btconly, noasan)	4m 23s
HWI test	0s
UI diff from main branch	2m 29s
	35m 18s

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