Use readString() to trim trailing LF from first line

Use readString() to trim trailing LF from first line

The fetch-pack/upload-pack stream usually has an LF at the
end of the first "want" line. Trim this when checking to
see if side-band or side-band-64k was used.

Perform the same trim for send-pack/receive-pack, as it is
harmless in this context to ignore an LF just before doing
an error report.

Change-Id: I6ef946bb6124fa72c52bd5320187eaac3ed906e7