tests: check error message from run_command

tests: check error message from run_command

In git versions starting at v1.7.5-rc0~29^2 until v1.7.5-rc3~2 (Revert
"run-command: prettify -D_FORTIFY_SOURCE workaround", 2011-04-18)
fixed it, the run_command facility would write a truncated error
message when the command is present but cannot be executed for some
other reason.  For example, if I add a 'hello' command to git:

	$ echo 'echo hello' >git-hello
	$ chmod +x git-hello
	$ PATH=.:$PATH git hello
	hello

and make it non-executable, this is what I normally get:

	$ chmod -x git-hello
	$ git hello
	fatal: cannot exec 'git-hello': Permission denied

But with the problematic versions, we get disturbing output:

	$ PATH=.:$PATH git hello
	fatal: $

Add some tests to make sure it doesn't happen again.

The hello-script used in these tests uses cat instead of echo because
on Windows the bash spawned by git converts LF to CRLF in text written
by echo while the bash running tests does not, causing the test to
fail if "echo" is used.  Thanks to Hannes for noticing.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Improved-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>