strbuf: add strbuf_getcwd()

strbuf: add strbuf_getcwd()

Add strbuf_getcwd(), which puts the current working directory into a
strbuf.  Because it doesn't use a fixed-size buffer it supports
arbitrarily long paths, provided the platform's getcwd() does as well.
At least on Linux and FreeBSD it handles paths longer than PATH_MAX
just fine.

Suggested-by: Karsten Blees <redacted>
Helped-by: Duy Nguyen <redacted>
Signed-off-by: Rene Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>