test: replace shebangs with descriptions in shell libraries

A #! line in these files is misleading, since these scriptlets are
meant to be sourced with '.' (using whatever shell sources them)
instead of run directly using the interpreter named on the #! line.

Removing the #! line shouldn't hurt syntax highlighting since
these files have filenames ending with '.sh'.  For documentation,
add a brief description of how the files are meant to be used in
place of the shebang line.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-read-tree.sh

@@ -1,5 +1,3 @@
-#!/bin/sh
-#
 # Helper functions to check if read-tree would succeed/fail as expected with
 # and without the dry-run option. They also test that the dry-run does not
 # write the index and that together with -u it doesn't touch the work tree.