checknlink: return False if .hgtmp file preexists (issue2517)
If os_link fails on Windows, errno is always errno.EINVAL,
so we can't really say if the testlink could not be created
because (a) the FS doesn't support hardlinks or (b) there
is a leaked .hgtmp file lying around from a previous crashed
run.
So let's err on the safe side, keep the code simple and assume
we can't detect hardlinks in both cases.
Test hangup signal in the middle of transaction
$ "$TESTDIR/hghave" fifo || exit 80
$ hg init
$ mkfifo p
$ hg serve --stdio < p &
$ P=$!
$ (echo lock; echo addchangegroup; sleep 5) > p &
$ Q=$!
$ sleep 3
0
0
adding changesets
$ kill -HUP $P
$ wait
transaction abort!
rollback completed
killed!
$ echo .hg/* .hg/store/*
.hg/00changelog.i .hg/journal.branch .hg/journal.desc .hg/journal.dirstate .hg/requires .hg/store .hg/store/00changelog.i .hg/store/00changelog.i.a