tests: avoid quoting of commit messages for test portability
journal extension uses util.shellquote() to record command line, but
result of it depends on runtime platform: double quotation is used on
Windows and OpenVMS, but single quotation is used otherwise.
test-journal-share.t sometimes specifies commit messages including
white space on command line. It makes journal output depend on runtime
platform, but commit message itself isn't important in this test case.