stash: convert apply to builtin

Add a builtin helper for performing stash commands. Converting
all at once proved hard to review, so starting with just apply
lets conversion get started without the other commands being
finished.

The helper is being implemented as a drop in replacement for
stash so that when it is complete it can simply be renamed and
the shell script deleted.

Delete the contents of the apply_stash shell function and replace
it with a call to stash--helper apply until pop is also
converted.

Signed-off-by: Joel Teichroeb <joel@teichroeb.net>
Signed-off-by: Paul-Sebastian Ungureanu <ungureanupaulsebastian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: klusark

.gitignore

@@ -162,6 +162,7 @@
 /git-show-ref
 /git-stage
 /git-stash
+/git-stash--helper
 /git-status
 /git-stripspace
 /git-submodule

Makefile

@@ -1138,6 +1138,7 @@ BUILTIN_OBJS += builtin/shortlog.o
 BUILTIN_OBJS += builtin/show-branch.o
 BUILTIN_OBJS += builtin/show-index.o
 BUILTIN_OBJS += builtin/show-ref.o
+BUILTIN_OBJS += builtin/stash--helper.o
 BUILTIN_OBJS += builtin/stripspace.o
 BUILTIN_OBJS += builtin/submodule--helper.o
 BUILTIN_OBJS += builtin/symbolic-ref.o

builtin.h

@@ -225,6 +225,7 @@ extern int cmd_show(int argc, const char **argv, const char *prefix);
 extern int cmd_show_branch(int argc, const char **argv, const char *prefix);
 extern int cmd_show_index(int argc, const char **argv, const char *prefix);
 extern int cmd_status(int argc, const char **argv, const char *prefix);
+extern int cmd_stash__helper(int argc, const char **argv, const char *prefix);
 extern int cmd_stripspace(int argc, const char **argv, const char *prefix);
 extern int cmd_submodule__helper(int argc, const char **argv, const char *prefix);
 extern int cmd_symbolic_ref(int argc, const char **argv, const char *prefix);