issue369: make it simpler to enter filenames.

Add .sfd to files without an extension of either sfd or sfdir
if that happens then place a warning in the log to that effect.
issue1344: work on source tree clones from bootstrap.

Some build environments clone the source tree before starting the
bootstrap/configure process. For example, homebrew on osx. That being
the case, some of these do not copy the .git tree in the clone
probably because it is not intended to be long lived. BUT if there is
no .git subdirectory then the old bootstrap would subtly fail by
forcing the GIT command to be out of bounds.

This update allows bootstrap to run in such .git'less clones and thus
removes one hurdle to FontForge building from homebrew.
Merge pull request #1590 from monkeyiq/2014/aug/osx-bootstrap-git-check-tree-copy-workaround

issue1344: work on source tree clones from bootstrap.
moved to the GFileNameTail() function, added the docs to that function
and included a you might have been looking for basename note to help
future users.
Merge pull request #1572 from monkeyiq/2014/aug/369

issue369: make it simpler to enter filenames.