Documentation, code and test clean-up around "git stash".
* dl/stash-cleanup:
stash: declare ref_stash as an array
t3905: use test_cmp() to check file contents
t3905: replace test -s with test_file_not_empty
t3905: remove nested git in command substitution
t3905: move all commands into test cases
t3905: remove spaces after redirect operators
git-stash.txt: be explicit about subcommand options