gitk: Refactor per-line part of getblobdiffline and its support

For later use with data sources other than a pipe, refactor the big
worker part of getblobdiffline to a separate function
parseblobdiffline.  Also refactor its initialization and wrap-up to
separate routines.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Paul Mackerras <paulus@samba.org>