use consistent names for the #include / #define header guard pattern.

Honor 'GIT_USE_NSEC' option in `filesystem_iterator_set_current`

Use longer conflict markers in recursive merge base

This should have been part of PR #3638. Without this we still get 
nsec-related errors, even when using -DGIT_USE_NSEC:

 error: ‘struct stat’ has no member named ‘st_mtime_nsec’

The null OID (hash with all zeroes) indicates a missing object in
upstream git and is thus not a valid object ID. Add defensive
measurements to avoid writing such a hash to the object database in the
very unlikely case where some data results in the null OID. Furthermore,
add shortcuts when reading the null OID from the ODB to avoid ever
returning an object when a faulty repository may contain the null OID.

In commit a96d3cc3f (cache-tree: reject entries with null sha1,
2017-04-21), the git.git project has changed its stance on null OIDs in
tree objects. Previously, null OIDs were accepted in tree entries to
help tools repair broken history. This resulted in some problems though
in that many code paths mistakenly passed null OIDs to be added to a
tree, which was not properly detected.

Align our own code base according to the upstream change and reject
writing tree entries early when the OID is all-zero.

Fixes #4492, #4496.

Git uses longer conflict markers in the recursive merge base - two more
than the default (thus, 9 character long conflict markers).  This allows
users to tell the difference between the recursive merge conflicts and
conflicts between the ours and theirs branches.

This was introduced in git d694a17986a28bbc19e2a6c32404ca24572e400f.

Update our tests to expect this as well.

Allow for a custom conflict marker size, allowing callers to override
the default size of the "<<<<<<<" and ">>>>>>>" markers in the
conflicted output file.