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-terminal.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+# Helpers for terminal output tests.
 
 test_expect_success PERL 'set up terminal for tests' '
 	# Reading from the pty master seems to get stuck _sometimes_