fast-import: disallow more path components #1198

Job	Run time
config	5s
fuzz smoke test	33s
win+VS build	0s
linux-musl (alpine)	6m 3s
linux32 (i386/ubuntu:focal)	6m 4s
pedantic (fedora)	1m 56s
documentation	3m 42s
linux-asan-ubsan (ubuntu-latest)	6m 16s
linux-gcc (ubuntu-20.04)	6m 15s
linux-gcc-default (ubuntu-latest)	6m 15s
linux-leaks (ubuntu-latest)	6m 13s
linux-reftable (ubuntu-latest)	6m 15s
linux-reftable-leaks (ubuntu-latest)	6m 14s
linux-sha256 (ubuntu-latest)	6m 14s
linux-TEST-vars (ubuntu-20.04)	6m 13s
osx-clang (macos-13)	6m 32s
osx-gcc (macos-13)	6m 32s
osx-reftable (macos-13)	6m 32s
sparse	3m 9s
win build	6m 8s
static-analysis	4m 20s
win+VS test	0s
win test	0s
	1h 41m 31s

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