t4150: test applying StGit patch

t4150: test applying StGit patch

By default, an StGit patch separates the subject from the commit message
and headers as follows:

	$subject

	From: $author_name <$author_email>

	$message
	---
	$diffstats

We test git-am's ability to detect such a patch as an StGit patch, and
its ability to be able to extract the commit author, date and message
from such a patch.

Based-on-patch-by: Chris Packham <redacted>
Signed-off-by: Paul Tan <redacted>
Signed-off-by: Junio C Hamano <redacted>