Set ORIG_HEAD when beginning to rebuild

This also quietens down the git-checkout command performed to move to
the integration branch and detach from it and removes a redundant extra
"detach HEAD" step.

Signed-off-by: John Keeping <john@keeping.me.uk>
Improve "reset to base" step output

Output a friendly "resetting to ..." message and make "reset --hard"
quieter to confuse the user less.

Signed-off-by: John Keeping <john@keeping.me.uk>
Improve error handling in do_merge

Signed-off-by: John Keeping <john@keeping.me.uk>
Print "Successfully re-integration ..." message to stdout

Also terminate the message with a full stop.

Signed-off-by: John Keeping <john@keeping.me.uk>
Improve merge output

Add an ellipsis to the "Merging branch ..." message and pass --quiet to
git-merge.  This makes the output of "git integration --rebuild" much
more concise and readable.

Signed-off-by: John Keeping <john@keeping.me.uk>