If the header doesn't look like a header (e.g. if it doesn't have a ":"
or if it has newlines), report "custom HTTP header '%s' is malformed".

If the header has the same name as a header already set by libgit2 (e.g.
"Host"), report "HTTP header '%s' is already set by libgit2".

Also, *some* custom headers actually are valid.

Provide path matching in the iterators (for faster diffs)

Some nicer refactoring for index iteration walks.

The index iterator doesn't binary search through the pathlist space,
since it lacks directory entries, and would have to binary search
each index entry and all its parents (eg, when presented with an index
entry of `foo/bar/file.c`, you would have to look in the pathlist for
`foo/bar/file.c`, `foo/bar` and `foo`).  Since the index entries and the
pathlist are both nicely sorted, we walk the index entries in lockstep
with the pathlist like we do for other iteration/diff/merge walks.

Now that non-pathspec matching diffs are implemented at the iterator
level, drop `FILELIST_MATCH`ing.

When using literal pathspecs in diff with `GIT_DIFF_DISABLE_PATHSPEC_MATCH`
turn on the faster iterator pathlist handling.

Updates iterator pathspecs to include directory prefixes (eg, `foo/`)
for compatibility with `GIT_DIFF_DISABLE_PATHSPEC_MATCH`.

Escape @ in doc comment

Document that `GIT_DIFF_PATHSPEC_DISABLE` is not necessarily about
explicit path matching, but also includes matching of directory
names.  Enforce this in a test.

win32: ensure hidden files can be staged

When given a pathlist, don't assume that directories sort before
files.  Walk through any list of entries sorting before us to make
sure that we've exhausted all entries that *aren't* directories.

Eg, if we're searching for 'foo/bar', and we have a 'foo.c', keep
advancing the pathlist to keep looking for an entry prefixed with
'foo/'.