test-lib: introduce test_line_count to measure files

Some tests check their output with code like the following:

	test "$(git ls-files -u B | wc -l)" -eq 3 || {
		echo "BAD: should have left stages for B"
		return 1
	}

The verbose failure condition is used because test, unlike
diff, does not print any useful information about the
nature of the failure when it fails.

Introduce a test_line_count function to help. If used like

	git ls-files -u B >output &&
	test_line_count -eq 3 output

it will produce output like

	test_line_count: line count for output !-eq 3
	100644 b023018cabc396e7692c70bbf5784a93d3f738ab 2	hi.c
	100644 45b983be36b73c0788dc9cbcb76cbb80fc7bb057 3	hi.c

on failure.

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

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