alias.c: reject too-long cmdline strings in split_cmdline()

This function improperly uses an int to represent the number of entries
in the resulting argument array. This allows a malicious actor to
intentionally overflow the return value, leading to arbitrary heap
writes.

Because the resulting argv array is typically passed to execv(), it may
be possible to leverage this attack to gain remote code execution on a
victim machine. This was almost certainly the case for certain
configurations of git-shell until the previous commit limited the size
of input it would accept. Other calls to split_cmdline() are typically
limited by the size of argv the OS is willing to hand us, so are
similarly protected.

So this is not strictly fixing a known vulnerability, but is a hardening
of the function that is worth doing to protect against possible unknown
vulnerabilities.

One approach to fixing this would be modifying the signature of
`split_cmdline()` to look something like:

    int split_cmdline(char *cmdline, const char ***argv, size_t *argc);

Where the return value of `split_cmdline()` is negative for errors, and
zero otherwise. If non-NULL, the `*argc` pointer is modified to contain
the size of the `**argv` array.

But this implies an absurdly large `argv` array, which more than likely
larger than the system's argument limit. So even if split_cmdline()
allowed this, it would fail immediately afterwards when we called
execv(). So instead of converting all of `split_cmdline()`'s callers to
work with `size_t` types in this patch, instead pursue the minimal fix
here to prevent ever returning an array with more than INT_MAX entries
in it.

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