memory_limit: use git_env_ulong() to parse GIT_ALLOC_LIMIT

memory_limit: use git_env_ulong() to parse GIT_ALLOC_LIMIT

GIT_ALLOC_LIMIT limits xmalloc()'s size, which is of type size_t.
Better use git_env_ulong() to parse the environment variable, so
that the postfixes 'k', 'm', and 'g' can be used; and use size_t to
store the limit for consistency.  The change to size_t has no direct
practical impact, because the environment variable is only meant to
be used for our own tests, and we use it to test small sizes.

The cast of size in the call to die() is changed to uintmax_t to
match the format string PRIuMAX.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>