completion: zsh: support completion after "git -C <path>" #28580

Job	Run time
config	14s
fuzz smoke test	1m 28s
win build	5m 0s
win+Meson build	3m 59s
osx-reftable (macos-14)	31m 45s
osx-meson (macos-14)	57m 33s
static-analysis	6m 37s
osx-clang (macos-14)	6h 0m 18s
documentation	7m 23s
rust-analysis	1m 31s
win+VS build	0s
osx-gcc (macos-14)	6h 0m 17s
sparse	3m 49s
linux-sha256 (ubuntu:rolling)	28m 57s
linux-breaking-changes (ubuntu:rolling)	28m 44s
linux32 (i386/ubuntu:20.04)	6h 0m 5s
linux-reftable (ubuntu:rolling)	24m 26s
linux-asan-ubsan (ubuntu:rolling)	1h 11m 29s
linux-TEST-vars (ubuntu:20.04)	27m 37s
linux-musl-meson (alpine:latest)	23m 42s
linux-leaks (ubuntu:rolling)	51m 40s
fedora-breaking-changes-meson (fedora:latest)	18m 1s
debian-11 (debian:11)	22m 5s
linux-reftable-leaks (ubuntu:rolling)	46m 40s
almalinux-8 (almalinux:8)	23m 30s
linux-meson (ubuntu:rolling)	25m 47s
win+VS test	-1s
win test (8)	5m 1s
win test (1)	6m 21s
win test (0)	10m 20s
win test (5)	16m 25s
win test (2)	8m 34s
win test (4)	58m 52s
win test (3)	11m 8s
win test (9)	5m 51s
win test (7)	7m 47s
win test (6)	10m 12s
win+Meson test (1)	6m 57s
win+Meson test (7)	8m 4s
win+Meson test (0)	8m 2s
win+Meson test (9)	7m 31s
win+Meson test (8)	10m 43s
win+Meson test (6)	14m 37s
win+Meson test (4)	9m 0s
win+Meson test (3)	10m 20s
win+Meson test (5)	9m 15s
win+Meson test (2)	59m 45s
	1d 7h 17m 21s

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