magit: Separate mode for magit-repos #866

Job	Run time
check (ubuntu-latest, 26.3, false)	48s
check (ubuntu-latest, 27.2, false)	46s
check (ubuntu-latest, 28.2, false)	1m 2s
check (ubuntu-latest, 29.3, false)	1m 0s
check (macos-latest, 28.2, false)	4m 1s
check (macos-latest, 29.3, false)	3m 44s
check (windows-latest, 26.3, false)	3m 23s
check (windows-latest, 27.2, false)	2m 18s
check (windows-latest, 28.2, false)	2m 11s
check (windows-latest, 29.3, false)	2m 11s
check (ubuntu-latest, snapshot, true)	43s
check (macos-latest, snapshot, true)	3m 51s
	25m 58s

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