commit: Handle a --use-log-file option.

commit: Handle a --use-log-file option.

It is now possible to prepare the ChangeLog entry to submit, put it in a
file and run `svn-wrapper.sh commit --use-log-file my-entry'.  The file
format used in `my-entry' is similar to what svn-wrapper usually asks you to
fill in.  Here is a sample of such file:

---------------------------------------------------------------------
Title: My change title.
Subject: r<REV>: <TITLE>
From: Some One <some.one@company.com>
To: tsuna

Comment:
  URL: http://path/to/repo

ChangeLog:

<YYYY>-<MM>-<DD>  Benoit Sigoure  <tsuna@lrde.epita.fr>

	<TITLE>
	* zog: New.
---------------------------------------------------------------------

The `Comment' section is optional.  The `ChangeLog' section must be present.
The `To:' line can be omitted if you don't want to send a mail.  The various
tags (<REV>, <TITLE>, <YYYY>, <MM>, <DD>, ...) are also optional, but will
be expanded if they're used.  You must provide a `Title' as well as a
`Subject', even if they end up being identical.

Finally, note that the --use-log-file must not appear before the name of the
sub-command (that is, `commit' or `ci') and that no short-option is provided
to avoid name clashes with the underlying SCM (be it Git or,  if you're
unlucky, SVN).

	* svn_commit: Handle that new option.
	* svn_help: Document it.

Signed-off-by: Benoit Sigoure <tsuna@lrde.epita.fr>