git-am foreign patch support: autodetect some patch formats

Default to mbox format if input is from stdin. Otherwise, look at the
first few lines of the first patch to try to guess its format.

Include checks for mailboxes, stgit patch series, stgit single patches
and hg patches.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-am.sh

@@ -142,7 +142,45 @@ check_patch_format () {
 	then
 		return 0
 	fi
-	patch_format=mbox
+
+	# we default to mbox format if input is from stdin and for
+	# directories
+	if test $# = 0 || test "x$1" = "x-" || test -d "$1"
+	then
+		patch_format=mbox
+		return 0
+	fi
+
+	# otherwise, check the first few lines of the first patch to try
+	# to detect its format
+	{
+		read l1
+		read l2
+		read l3
+		case "$l1" in
+		"From "* | "From: "*)
+			patch_format=mbox
+			;;
+		'# This series applies on GIT commit'*)
+			patch_format=stgit-series
+			;;
+		"# HG changeset patch")
+			patch_format=hg
+			;;
+		*)
+			# if the second line is empty and the third is
+			# a From, Author or Date entry, this is very
+			# likely an StGIT patch
+			case "$l2,$l3" in
+			,"From: "* | ,"Author: "* | ,"Date: "*)
+				patch_format=stgit
+				;;
+			*)
+				;;
+			esac
+			;;
+		esac
+	} < "$1"
 }
 
 split_patches () {