doc: add more folks to the "Thanks to" file

Signed-off-by: David Aguilar <davvid@gmail.com>
main.model: remove redundant function

unstage_all() == reset() with no arguments.  Keep reset().
unstage_all() was also passing '.', which is not needed.

Signed-off-by: David Aguilar <davvid@gmail.com>
gitcmds: support repositories with files named "HEAD"

`git diff-index HEAD` and `git log HEAD` both error out when a file
named "HEAD" exists in the repository:

	$ git diff-index HEAD
	fatal: ambiguous argument 'HEAD': both revision and filename
	Use '--' to separate filenames from revisions

Make our use of git more robust by using the '--' separator when asking
for "HEAD".

Closes #177

Reported-by: Maarten Nieber @mnieber via github.com
Signed-off-by: David Aguilar <davvid@gmail.com>