Support patches without a prefix by determining the prefix depth #28250

Job	Run time
Coq 8.16.1	7m 28s
Coverage (5.1.x)	9m 33s
Build microbenchmarks	1m 33s
Build (macos-latest, 5.1.x)	13m 36s
Build (ubuntu-latest, 5.1.x)	9m 10s
Documentation	1m 14s
Build (ocaml-variants.5.3.0+trunk, ubuntu-latest, true)	4m 57s
Create local opam switch (macos-latest, 5)	5m 53s
Format	1m 14s
Build (4.14.x, ubuntu-latest, true)	1m 39s
Create local opam switch (macos-latest, 4.14)	7m 6s
Build (4.14.x, macos-latest, true)	2m 18s
Build monorepo benchmark docker image	3m 49s
Create local opam switch (ubuntu-latest, 5)	5m 5s
Build (5.1.x, macos-13, true)	4m 16s
Create local opam switch (ubuntu-latest, 4.14)	4m 52s
Nix (macos-latest)	2m 1s
Build (ocaml-base-compiler.4.14.2,system-msvc, windows-latest)	16m 7s
Nix (ubuntu-latest)	55s
Build (ocaml-base-compiler.5.1.1,system-mingw, windows-latest)	10m 16s
Build (4.08.x, ubuntu-latest, true)	2m 1s
Build (4.04.x, ubuntu-latest, true, true)	6m 11s
Build (4.02.x, ubuntu-latest, true)	4m 3s
	2h 5m 17s

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