Make trailing LF following fast-import `data` commands optional

Make trailing LF following fast-import `data` commands optional

A few fast-import frontend developers have found it odd that we
require the LF following a `data` command, especially in the exact
byte count format.  Technically we don't need this LF to parse
the stream properly, but having it here does make the stream more
readable to humans.  We can easily make the LF optional by peeking
at the next byte available from the stream and pushing it back into
the buffer if its not LF.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>