taskd/clone.sh: trim excessively large successful .clonelog files
If a successful clone's .clonelog file exceeds 10000 lines, trim
it back to the first 5000 lines plus a comment about how many lines
have been omitted plus the last 5000 lines.
Some very long running clones, especially git-svn clones, can produce
multi-megabyte-sized .clonelog files that are not particularly
useful, especially after a successful clone.
Retaining the first and last 5000 lines of such a log file should
more than suffice for a successful clone.
For a failed clone, the .clonelog file remains untouched.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>