t4150: test applying StGit series

t4150: test applying StGit series

A StGit series is a directory containing a "series" file which begins
with the line:

	# This series applies on GIT commit XXXXX

where XXXXX is the commit ID that the patch series applies on. Every
following line names a patch in the directory to be applied.

Test that git-am, when given this "series" file, is able to detect it as
an StGit series and apply all the patches in the series.

Signed-off-by: Paul Tan <redacted>
Signed-off-by: Junio C Hamano <redacted>