git-am: fixed patch_format detection according to RFC2822

RFC2822 specifies in paragraph 3.6.8, that optional header fields are
made up of any printable US-ASCII character except ' ' (space) and ':'
(colon).

The pattern for the egrep command is changed to match all of these
characters.
Signed-off-by: NChristian Himpel <chressie@gmail.com>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

git-am.sh

@@ -205,7 +205,7 @@ check_patch_format () {
 			# and see if it looks like that they all begin with the
 			# header field names...
 			sed -n -e '/^$/q' -e '/^[ 	]/d' -e p "$1" |
-			egrep -v '^[A-Za-z]+(-[A-Za-z]+)*:' >/dev/null ||
+			egrep -v '^[!-9;-~]+:' >/dev/null ||
 			patch_format=mbox
 		fi
 	} < "$1" || clean_abort