Fix shell quoting issues with the commit messages

Fix shell quoting issues with the commit messages

The commit message was subject to shell double quote expansion because
it was passed as a parameter to `ci' and external commands are executed
via a shell.  This could cause commit messages to be modified and to
even fail the import.  Single quote the commit message instead.

Test case:

$ cat test-03-comment-quoting.fastexp
commit refs/heads/master
mark :1
committer Joe Bloggs <joe.bloggs@example.com> 998655365 +0100
data 84
Commit message containing shell double quote special characters (`) ($HOME) and (')

M 644 inline file.txt
data 21
Contents of file.txt

$ rcs-fast-export -v -v -v < test-03-comment-quoting.fastexp
rcs-fast-import: repository fast import creates
/tmp/test/.rs24387
rcs-fast-import:......(0.00 sec) done.
commit refs/heads/master
rcs-fast-import:    :1 M file.txt
rcs-fast-import: linking ../.rs24387/inline-152187820 to file.txt
Assigning 1.1 to file.txt
rcs-fast-import: executing 'ci -l1.1 -d'Fri 24 Aug 2001 13:16:05 +0100'
-m"Committer: Joe Bloggs <joe.bloggs@example.com>
Committer-Date: Fri 24 Aug 2001 13:16:05 +0100
Mark: :1

Commit message containing shell double quote special characters (`) ($HOME) and (')
" file.txt RCS/file.txt,v' modify
/bin/sh: -c: line 4: unexpected EOF while looking for matching ``'
/bin/sh: -c: line 6: syntax error: unexpected end of file
cleaning up......(0.00 sec) done.
rcs-fast-import: cleaning up
/tmp/test/.rs24387......(0.00 sec)
done.
rcs-fast-import: child returned 1.