git-am foreign patch support: introduce patch_format

Set up a framework to allow git-am to support patches which are not in
mailbox format. Introduce a patch_format variable that presently can
only be set from the command line, defaulting to 'mbox' (the only
supported format) if not specified.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -18,6 +18,7 @@ whitespace=     pass it through git-apply
 directory=      pass it through git-apply
 C=              pass it through git-apply
 p=              pass it through git-apply
+patch-format=   format the patch(es) are in
 reject          pass it through git-apply
 resolvemsg=     override error message when patch failure occurs
 r,resolved      to be used after a patch failure
@@ -133,6 +134,32 @@ It does not apply to blobs recorded in its index."
     unset GITHEAD_$his_tree
 }
 
+patch_format=
+
+check_patch_format () {
+	# early return if patch_format was set from the command line
+	if test -n "$patch_format"
+	then
+		return 0
+	fi
+	patch_format=mbox
+}
+
+split_patches () {
+	case "$patch_format" in
+	mbox)
+		git mailsplit -d"$prec" -o"$dotest" -b -- "$@" > "$dotest/last" ||  {
+			rm -fr "$dotest"
+			exit 1
+		}
+		;;
+	*)
+		echo "Patch format $patch_format is not supported."
+		exit 1
+		;;
+	esac
+}
+
 prec=4
 dotest="$GIT_DIR/rebase-apply"
 sign= utf8=t keep= skip= interactive= resolved= rebasing= abort=
@@ -175,6 +202,8 @@ do
 		git_apply_opt="$git_apply_opt $(sq "$1=$2")"; shift ;;
 	-C|-p)
 		git_apply_opt="$git_apply_opt $(sq "$1$2")"; shift ;;
+	--patch-format)
+		shift ; patch_format="$1" ;;
 	--reject)
 		git_apply_opt="$git_apply_opt $1" ;;
 	--committer-date-is-author-date)
@@ -274,10 +303,10 @@ else
 		done
 		shift
 	fi
-	git mailsplit -d"$prec" -o"$dotest" -b -- "$@" > "$dotest/last" ||  {
-		rm -fr "$dotest"
-		exit 1
-	}
+
+	check_patch_format "$@"
+
+	split_patches "$@"
 
 	# -s, -u, -k, --whitespace, -3, -C and -p flags are kept
 	# for the resuming session after a patch failure.