[PATCH v2 0/6] Convert some stash functionality to a builtin

I've been working on converting all of git stash to be a builtin, however it's hard to get it all working at once with limited time, so I've moved around half of it to a new stash--helper builtin and called these functions from the shell script. Once this is stabalized, it should be easier to convert the rest of the commands one at a time without breaking anything.

I've sent most of this code before, but that was targetting a full replacement of stash. The code is overall the same, but with some code review changes and updates for internal api changes.

Since there seems to be interest from GSOC students who want to work on converting builtins, I figured I should finish what I have that works now so they could build on top of it.

Previous threads: v1: https://public-inbox.org/git/20180325173916.GE10909@hank/T/

Changes from v1:
 - Fixed memory leaks
 - Fixed formatting
 - Fixed typos
 - Removed old .sh code that has been replaced
 - Fix differences in error messages
 - Added tests to ensure argument handling work when specifiying
   multiple refs, or no arguments for branch
 - Refactored the code a bit
 - Reordered the patches so they compile and test successfully

Joel Teichroeb (6):
  stash: Add tests for passing in too many refs
  stash: Add test for branch with no arguments
  stash: convert apply to builtin
  stash: convert drop and clear to builtin
  stash: convert branch to builtin
  stash: convert pop to builtin

 .gitignore              |   1 +
 Makefile                |   1 +
 builtin.h               |   1 +
 builtin/stash--helper.c | 620 ++++++++++++++++++++++++++++++++++++++++++++++++
 git-stash.sh            | 125 +---------
 git.c                   |   1 +
 t/t3903-stash.sh        |  26 ++
 7 files changed, 658 insertions(+), 117 deletions(-)
 create mode 100644 builtin/stash--helper.c