abspath: convert absolute_path() to strbuf

abspath: convert absolute_path() to strbuf

Move most of the code of absolute_path() into the new function
strbuf_add_absolute_path() and in the process transform it to use
struct strbuf and xgetcwd() instead of a PATH_MAX-sized buffer,
which can be too small on some file systems.

Signed-off-by: Rene Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>