vcs-svn: replace buffer_read_string memory pool with a strbuf

vcs-svn: replace buffer_read_string memory pool with a strbuf

obj_pool is inherently global and does not use the standard growing
factor alloc_nr, which makes it feel out of place in the git codebase.
Plus it is overkill for this application: all that is needed is a
buffer that can grow between requests to accomodate larger strings.
Use a strbuf instead.

As a side effect, this improves the error handling: allocation
failures will result in a clean exit instead of segfaults.  It would
be nice to add a test case (using ulimit or failmalloc) but that can
wait for another day.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>