rebase -i: style fixes and minor cleanups

rebase -i: style fixes and minor cleanups

This patch indents ";;" consistently with the rest of git's shell scripts,
and makes sure that ";;" are before each "esac".

It introduces a helper function "has_action", to make it easier to read
the intentions of the code.

Errors from "git rev-parse --verify" are no longer ignored.

Spaces are quoted using single quotes instead of a backslash, for
readability.

A "test $preserve=f" (missing spaces) was fixed; hashes are no longer
written to "$DOTEST"/rewritten/ unnecessarily.

We used to quote the message for a squash, only to have "echo" unquote it.
Now we use "printf" and do not need to quote to start with.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>