shell: limit size of interactive commands

When git-shell is run in interactive mode (which must be enabled by
creating $HOME/git-shell-commands), it reads commands from stdin, one
per line, and executes them.

We read the commands with git_read_line_interactively(), which uses a
strbuf under the hood. That means we'll accept an input of arbitrary
size (limited only by how much heap we can allocate). That creates two
problems:

  - the rest of the code is not prepared to handle large inputs. The
    most serious issue here is that split_cmdline() uses "int" for most
    of its types, which can lead to integer overflow and out-of-bounds
    array reads and writes. But even with that fixed, we assume that we
    can feed the command name to snprintf() (via xstrfmt()), which is
    stuck for historical reasons using "int", and causes it to fail (and
    even trigger a BUG() call).

  - since the point of git-shell is to take input from untrusted or
    semi-trusted clients, it's a mild denial-of-service. We'll allocate
    as many bytes as the client sends us (actually twice as many, since
    we immediately duplicate the buffer).

We can fix both by just limiting the amount of per-command input we're
willing to receive.

We should also fix split_cmdline(), of course, which is an accident
waiting to happen, but that can come on top. Most calls to
split_cmdline(), including the other one in git-shell, are OK because
they are reading from an OS-provided argv, which is limited in practice.
This patch should eliminate the immediate vulnerabilities.

I picked 4MB as an arbitrary limit. It's big enough that nobody should
ever run into it in practice (since the point is to run the commands via
exec, we're subject to OS limits which are typically much lower). But
it's small enough that allocating it isn't that big a deal.

The code is mostly just swapping out fgets() for the strbuf call, but we
have to add a few niceties like flushing and trimming line endings. We
could simplify things further by putting the buffer on the stack, but
4MB is probably a bit much there. Note that we'll _always_ allocate 4MB,
which for normal, non-malicious requests is more than we would before
this patch. But on the other hand, other git programs are happy to use
96MB for a delta cache. And since we'd never touch most of those pages,
on a lazy-allocating OS like Linux they won't even get allocated to
actual RAM.

The ideal would be a version of strbuf_getline() that accepted a maximum
value. But for a minimal vulnerability fix, let's keep things localized
and simple. We can always refactor further on top.

The included test fails in an obvious way with ASan or UBSan (which
notice the integer overflow and out-of-bounds reads). Without them, it
fails in a less obvious way: we may segfault, or we may try to xstrfmt()
a long string, leading to a BUG(). Either way, it fails reliably before
this patch, and passes with it. Note that we don't need an EXPENSIVE
prereq on it. It does take 10-15s to fail before this patch, but with
the new limit, we fail almost immediately (and the perl process
generating 2GB of data exits via SIGPIPE).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
