git-am: fix detection of absolute paths for windows

Add an is_absolute_path function to abstract out platform differences
in checking for an absolute or relative path.
Specifically fixes t4150-am on Windows.

[PT: updated following suggestion from j6t to support \* and //*]

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>

git-am.sh

@@ -444,12 +444,12 @@ else
 				set x
 				first=
 			}
-			case "$arg" in
-			/*)
-				set "$@" "$arg" ;;
-			*)
-				set "$@" "$prefix$arg" ;;
-			esac
+			if is_absolute_path "$arg"
+			then
+				set "$@" "$arg"
+			else
+				set "$@" "$prefix$arg"
+			fi
 		done
 		shift
 	fi

git-sh-setup.sh

@@ -209,5 +209,20 @@ case $(uname -s) in
 	find () {
 		/usr/bin/find "$@"
 	}
+	is_absolute_path () {
+		case "$1" in
+		[/\\]* | [A-Za-z]:*)
+			return 0 ;;
+		esac
+		return 1
+	}
 	;;
+*)
+	is_absolute_path () {
+		case "$1" in
+		/*)
+			return 0 ;;
+		esac
+		return 1
+	}
 esac