Fix t4200-rerere for white-space from "wc -l"

Fix t4200-rerere for white-space from "wc -l"

On OS X, wc outputs 6 spaces before the number of lines, so the test
expecting the string "10" failed.  Do not quote $cmd to strip away
the problematic whitespace as other tests do.

Also fix the grammar of the test name while making changes to it.
There's only one preimage, so it's "has", not "have".

Signed-off-by: Junio C Hamano <redacted>