diff.c: factor advance_or_nullify out of mark_color_as_moved

diff.c: factor advance_or_nullify out of mark_color_as_moved

This moves the part of code that checks if we're still in a block
into its own function.  We'll need a different approach on advancing
the blocks in a later patch, so having it as a separate function will
prove useful.

While at it rename the variable `p` to `prev` to indicate that it refers
to the previous line. This is as pmb[i] was assigned in the last iteration
of the outmost for loop.

Further rename `pnext` to `cur` to indicate that this should match up with
the current line of the outmost for loop.

Also replace the advancement of pmb[i] to reuse `cur` instead of
using `p->next` (which is how the name for pnext could be explained.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>