apply: force matching at the beginning.

apply: force matching at the beginning.

When there is no leading context, the patch must match at the
beginning of preimage; otherwise there is a "patch adds these
lines while the other lines were added to the original file"
conflict.

This is the opposite of match_end fix earlier in this series.
Unlike matching at the end case, we can additionally check the
preimage line number recorded in the patch, so the change is not
symmetrical with the earlier one.

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