t/helper: ignore everything but sources

Compiled test helpers in t/helper are out of sync with the .gitignore
files quite frequently. This can happen when new test helpers are added,
but the explicit .gitignore file is not updated in the same commit, or
when you forget to 'make clean' before checking out a different version
of git, as the different version may have a different explicit list of
test helpers to ignore.

Fix this by having an overly broad ignore pattern in that directory:
Anything, except C and shell source, will be ignored.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/.gitignore

@@ -1,38 +1,4 @@
-/test-chmtime
-/test-ctype
-/test-config
-/test-date
-/test-delta
-/test-dump-cache-tree
-/test-dump-split-index
-/test-dump-untracked-cache
-/test-fake-ssh
-/test-scrap-cache-tree
-/test-genrandom
-/test-hashmap
-/test-index-version
-/test-lazy-init-name-hash
-/test-line-buffer
-/test-match-trees
-/test-mergesort
-/test-mktemp
-/test-online-cpus
-/test-parse-options
-/test-path-utils
-/test-prio-queue
-/test-read-cache
-/test-ref-store
-/test-regex
-/test-revision-walking
-/test-run-command
-/test-sha1
-/test-sha1-array
-/test-sigchain
-/test-strcmp-offset
-/test-string-list
-/test-submodule-config
-/test-subprocess
-/test-svn-fe
-/test-urlmatch-normalization
-/test-wildmatch
-/test-write-cache
+*
+!*.sh
+!*.[ch]
+!*.gitignore