Make git-pull and git-am builtins

by Paul Tan for Git

git-pull and git-am are frequently used git subcommands. However, they are porcelain commands and implemented as shell scripts, which has some limitations which can cause poor performance, especially in non-POSIX environments like Windows. I propose to rewrite these scripts into low level C code and make them builtins. This will increase git's portability, and may improve the efficiency and performance of these commands.