Introduce some tests that show some commits, while hiding some commits
that have a timestamp older than the common ancestors of these two
commits.

Test a rebase (both a merge rebase and an inmemory rebase) with a new
commit that adds files underneath a new subfolder.

Use legitimate (existing) object IDs in tests so that we have the
ability to turn on strict object validation when running tests.

Introduce a repository that contains some paths that were illegal
on PC-DOS circa 1981 (like `aux`, `con`, `com1`) and that in a
bizarre fit of retrocomputing, remain illegal on some "modern"
computers, despite being "new technology".

Introduce some aspirational tests that suggest that we should be
able to cope with trees and indexes that contain paths that
would be illegal on the filesystem, so that we can at least diff
them.  Further ensure that checkout will not write a repository
with forbidden paths.

Line count overflow in git_blame_hunk and git_blame__entry

When formatting a patch as email we do not include the commit's
message in the formatted patch output. Implement this and add a
test that verifies behavior.

It is not unreasonable to have versioned files with a line count
exceeding 2^16. Upon blaming such files we fail to correctly keep
track of the lines as `git_blame_hunk` stores them in `uint16_t`
fields.

Fix this by converting the line fields of `git_blame_hunk` to
`size_t`. Add test to verify behavior.