update-ref --stdin: read the whole input at once

update-ref --stdin: read the whole input at once

Read the whole input into a strbuf at once, and then parse it from
there.  This might also be a tad faster, but that is not the point.
The point is to decouple the parsing code from the input source (the
old parsing code had to read new data even in the middle of commands).
Add docstrings for the parsing functions.

Signed-off-by: Michael Haggerty <redacted>
Signed-off-by: Junio C Hamano <redacted>