Support delimited data regions in fast-import.

Support delimited data regions in fast-import.

During testing its nice to not have to feed the length of a data
chunk to the 'data' command of fast-import.  Instead we would
prefer to be able to establish a data chunk much like shell's <<
operator and use a line delimiter to denote the end of the input.

So now if a data command is started as 'data <<EOF' we will look
for a terminator line containing only the string EOF on that line.
Once found, we stop the data command.  Everything between the two
lines is used as the data value.

The 'data <<' syntax is slower than 'data n', as we don't know how
many bytes to expect and instead must grow our buffer on the fly.
It also has the problem that the frontend must use a string which
will not appear on a line by itself in the input, and the data
region will always end in an LF.  For these reasons real import
frontends are encouraged to continue to use _only_ 'data n'.

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