Reimplement 'git pull' in C.

* pt/pull-builtin:
  pull: remove redirection to git-pull.sh
  pull --rebase: error on no merge candidate cases
  pull --rebase: exit early when the working directory is dirty
  pull: configure --rebase via branch.<name>.rebase or pull.rebase
  pull: teach git pull about --rebase
  pull: set reflog message
  pull: implement pulling into an unborn branch
  pull: fast-forward working tree if head is updated
  pull: check if in unresolved merge state
  pull: support pull.ff config
  pull: error on no merge candidates
  pull: pass git-fetch's options to git-fetch
  pull: pass git-merge's options to git-merge
  pull: pass verbosity, --progress flags to fetch and merge
  pull: implement fetch + merge
  pull: implement skeletal builtin pull
  argv-array: implement argv_array_pushv()
  parse-options-cb: implement parse_opt_passthru_argv()
  parse-options-cb: implement parse_opt_passthru()

GSoC project to rebuild ref listing by branch and tag based on the
for-each-ref machinery.  This is its first part.

* kn/for-each-ref:
  ref-filter: make 'ref_array_item' use a FLEX_ARRAY for refname
  for-each-ref: introduce filter_refs()
  ref-filter: move code from 'for-each-ref'
  ref-filter: add 'ref-filter.h'
  for-each-ref: rename variables called sort to sorting
  for-each-ref: rename some functions and make them public
  for-each-ref: introduce 'ref_array_clear()'
  for-each-ref: introduce new structures for better organisation
  for-each-ref: rename 'refinfo' to 'ref_array_item'
  for-each-ref: clean up code
  for-each-ref: extract helper functions out of grab_single_ref()

Move most of the code from 'for-each-ref' to 'ref-filter' to make
it publicly available to other commands, this is to unify the code
of 'tag -l', 'branch -l' and 'for-each-ref' so that they can share
their implementations with each other.

Add 'ref-filter' to the Makefile, this completes the movement of code
from 'for-each-ref' to 'ref-filter'.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

"git checkout [<tree-ish>] <paths>" spent unnecessary cycles
checking if the current branch was checked out elsewhere, when we
know we are not switching the branches ourselves.

* nd/multiple-work-trees:
  worktree: new place for "git prune --worktrees"
  checkout: don't check worktrees when not necessary

Clarify in the Makefile a guideline to decide use of USE_NSEC.

* kb/use-nsec-doc:
  Makefile / racy-git.txt: clarify USE_NSEC prerequisites

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Commit 23af91d (prune: strategies for linked checkouts - 2014-11-30)
adds "--worktrees" to "git prune" without realizing that "git prune" is
for object database only. This patch moves the same functionality to a
new command "git worktree".

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>

At the beginning of the rewrite of git-pull.sh to C, we introduced a
redirection to git-pull.sh if the environment variable
_GIT_USE_BUILTIN_PULL was not defined in order to not break test scripts
that relied on a functional git-pull.

Now that all of git-pull's functionality has been re-implemented in
builtin/pull.c, remove this redirection, and retire the old git-pull.sh
into contrib/examples/.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Build clean-up.

* jk/make-fix-dependencies:
  Makefile: silence perl/PM.stamp recipe
  Makefile: avoid timestamp updates to GIT-BUILD-OPTIONS
  Makefile: drop dependency between git-instaweb and gitweb

For the purpose of rewriting git-pull.sh into a C builtin, implement a
skeletal builtin/pull.c that redirects to $GIT_EXEC_PATH/git-pull.sh if
the environment variable _GIT_USE_BUILTIN_PULL is not defined. This
allows us to fall back on the functional git-pull.sh when running the
test suite for tests that depend on a working git-pull implementation.

This redirection should be removed when all the features of git-pull.sh
have been re-implemented in builtin/pull.c.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>