Add GIT_DIFF_LINE_CONTEXT_EOFNL

This adds a new line origin constant for the special line that
is used when both files end without a newline.

In the course of writing the tests for this, I was having problems
with modifying a file but not having diff notice because it was
the same size and modified less than one second from the start of
the test, so I decided to start working on nanosecond timestamp
support.  This commit doesn't contain the nanosecond support, but
it contains the reorganization of maybe_modified and the hooks so
that if the nanosecond data were being read by stat() (or rather
being copied by git_index_entry__init_from_stat), then the nsec
would be taken into account.

This new stuff could probably use some more tests, although there
is some amount of it here.
More tests for files with no newline at end
Improve diff function docs
Fix line numbering for patches with eofnl

When a patch contained an eofnl change (i.e. the last line either
gained or lost a newline), the oldno and newno line number values
for the lines in the last hunk of the patch were not useful.  This
makes them behave in a more expected manner.
Merge pull request #1556 from arrbee/diff-patch-fixes

Diff patch bug fixes