test-lib functions: expand "test_commit" comment template

Expand the comment template for "test_commit" to match that of
"test_commit_bulk" added in b1c36cb (test-lib: introduce
test_commit_bulk, 2019-07-02). It has several undocumented options,
which won't all fit on one line. Follow-up commit(s) will document
them.

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

t/test-lib-functions.sh

@@ -178,16 +178,14 @@ debug () {
 	GIT_DEBUGGER="${GIT_DEBUGGER}" "$@" <&6 >&5 2>&7
 }
 
-# Call test_commit with the arguments
-# [-C <directory>] <message> [<file> [<contents> [<tag>]]]"
+# Usage: test_commit [options] <message> [<file> [<contents> [<tag>]]]
+#   -C <dir>:
+#	Run all git commands in directory <dir>
 #
 # This will commit a file with the given contents and the given commit
 # message, and tag the resulting commit with the given tag name.
 #
 # <file>, <contents>, and <tag> all default to <message>.
-#
-# If the first argument is "-C", the second argument is used as a path for
-# the git invocations.
 
 test_commit () {
 	notick= &&