Support before-commit hook option (--pre-commit)

Support before-commit hook option (--pre-commit)

Allow the caller to optionally run a shell script before each
replayed commit. This could be useful if you wanted to modify the
replayed file/directory contents, e.g. strip-out a certain file at a
certain source revision, etc.

* svn2svn/run/svnreplay.py (commit_from_svn_log_entry): Support
  beforecommit hook.
* tests/t0103*: Add unit-tests for new --pre-commit option.

* svn2svn/run/svnreplay.py (main): Minor tweaks to command-line
  argument help-text.
* svn2svn/shell.py (_run_raw_command): Fix typo