More diff rename tests; better split swap handling

This adds a couple more tests of different rename scenarios.

Also, this fixes a problem with the case where you have two
"split" deltas and the left half of one matches the right half of
the other.  That case was already being handled, but in the wrong
order in a way that could result in bad output.  Also, if the swap
also happened to put the other two halves into the correct place
(i.e. two files exchanged places with each other), then the second
delta was left with the SPLIT flag set when it really should be
cleared.
Move core.abbrev lookup out of diff print loop

This moves the GIT_CVAR_ABBREV lookup out of the loop.  Also, this
fixes git_diff_print_raw to actually use that constant instead of
hardcoding 7 characters.