t5403: simplify by using a single repository

t5403: simplify by using a single repository

There is no strong reason to use separate clones to run
these tests; just use a single test repository prepared
with more modern test_commit shell helper function.

While at it, replace three "awk '{print $N}'" on the same
file with shell built-in "read" into three variables.

Revert d42ec126aa717d00549e387d5a95fd55683c2e2c which is a workaround for
Cygwin that is no longer needed.

Signed-off-by: Orgad Shaneh <redacted>
Acked-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>