fast-import: disallow "." and ".." path components #1194

Job	Run time
config	5s
fuzz smoke test	31s
win+VS build	0s
sparse	2m 33s
linux32 (i386/ubuntu:focal)	1m 31s
linux-musl (alpine)	1m 29s
pedantic (fedora)	1m 34s
win+VS test	0s
static-analysis	2m 32s
documentation	2m 31s
win build	2m 16s
linux-asan-ubsan (ubuntu-latest)	1m 43s
linux-gcc (ubuntu-20.04)	2m 32s
linux-gcc-default (ubuntu-latest)	2m 31s
linux-leaks (ubuntu-latest)	1m 42s
linux-reftable (ubuntu-latest)	2m 32s
linux-reftable-leaks (ubuntu-latest)	2m 32s
linux-sha256 (ubuntu-latest)	1m 42s
linux-TEST-vars (ubuntu-20.04)	2m 32s
osx-clang (macos-13)	2m 51s
osx-gcc (macos-13)	2m 53s
osx-reftable (macos-13)	1m 58s
win test	0s
	40m 30s

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