t/README: document test_grep helper

test_grep is a wrapper around grep for test assertions that prints
the file contents on failure for easier debugging.  It also accepts
'!' as its first argument for negation, which preserves the
diagnostic output that '! test_grep' would suppress.

Despite being widely used (and the preferred replacement for bare
grep in assertions), test_grep has no entry in t/README alongside
the other documented helpers like test_cmp and test_line_count.
Add one.

Signed-off-by: Michael Montalbo <mmontalbo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/README

@@ -1039,6 +1039,27 @@ see test-lib-functions.sh for the full list and their options.
 
    Check whether a file has the length it is expected to.
 
+ - test_grep [!] [<grep-options>] <pattern> <file>
+
+   Check whether <file> contains a line matching <pattern>, or
+   with '!' that no line matches.  Use this instead of bare
+   'grep <pattern> <file>' in test assertions.  On failure,
+   test_grep prints the contents of <file> for easier debugging,
+   whereas a bare 'grep' would fail silently.
+
+   For negation, pass '!' as the first argument:
+
+	test_grep ! "^diff --git" actual
+
+   Do not negate by writing '! test_grep', as that suppresses the
+   diagnostic output.
+
+   test_grep should only be used as a test assertion.  When grep
+   is used as a data filter (e.g. 'grep -v "^index" actual >filtered')
+   or inside a command substitution (e.g. '$(grep -c ...)'), plain
+   'grep' is the right choice because the exit code is not the
+   assertion itself.
+
  - test_path_is_file <path>
    test_path_is_dir <path>
    test_path_is_missing <path>