Better temp dir and output

[joshrabinowitz]

Note

• new use of git check-ignore -q ,
• removal of directory creation/destruction in _secrets_dir_is_not_ignored

Also relates to #407 and #360 as it sets TMPDIR (and thus stops BATS_TMPDIR from using the default /tmp).

[joshrabinowitz]

This PR seems to occasionally (like one build out of 10) fail one or two Travis tests. Here's some evidence, both from Ubuntu:

https://travis-ci.org/sobolevn/git-secret/jobs/509954140 - gnupg1-ubuntu-latest

               "not ok 30 run 'hide' normally with SECRETS_VERBOSE=1", 
               "# (from function `install_fixture_key' in file /tmp/git-secret/tests/_test_base.bash, line 105,", 
               "#  from function `setup' in test file /tmp/git-secret/tests/test_hide.bats, line 10)", 
               "#   `install_fixture_key \"$TEST_DEFAULT_USER\"' failed with status 2", 
               "# rm: cannot remove 'space file': No such file or directory", 
               "# fatal: not a git repository (or any of the parent directories): .git", 

               "not ok 62 run 'remove' with multiple arguments", 
               "# (from function `install_fixture_key' in file /tmp/git-secret/tests/_test_base.bash, line 105,", 
               "#  from function `setup' in test file /tmp/git-secret/tests/test_remove.bats, line 10)", 
               "#   `install_fixture_key \"$TEST_DEFAULT_USER\"' failed with status 2", 
               "# rm: cannot remove 'space file': No such file or directory", 
               "# rm: cannot remove 'space file two': No such file or directory", 
               "# fatal: not a git repository (or any of the parent directories): .git", 

And https://travis-ci.org/sobolevn/git-secret/jobs/509954149 - "gnupg-git-ubuntu-rolling"

               "not ok 23 run 'clean' with '-v'", 
               "# (from function `install_fixture_key' in file /tmp/git-secret/tests/_test_base.bash, line 105,", 
               "#  from function `setup' in test file /tmp/git-secret/tests/test_clean.bats, line 13)", 
               "#   `install_fixture_key \"$TEST_DEFAULT_USER\"' failed with status 2", 
               "# rm: cannot remove 'space file': No such file or directory", 
               "# rm: cannot remove 'space file two': No such file or directory", 
               "# rm: cannot remove 'somedir': No such file or directory", 
               "# fatal: not a git repository (or any of the parent directories): .git", 

[joshrabinowitz]

We have split this PR into 3 parts, see #440, #441, #442