subrepo: use subprocess's cwd instead of git's --work-tree
Some older git commands (e.g. git merge) do not properly recognize the
--work-tree argument, so abstract that away from git.
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