"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>

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

Group list of commands shown by "git help" along the workflow
elements to help early learners.

* sg/help-group:
  help: respect new common command grouping
  command-list.txt: drop the "common" tag
  generate-cmdlist: parse common group commands
  command-list.txt: add the common groups block
  command-list: prepare machinery for upcoming "common groups" section

Every time we run "make", we update perl/PM.stamp, which
contains a list of all of the perl module files (if it's
updated, we need to rebuild perl/perl.mak, since the
Makefile will not otherwise know about the new files).

This means that every time "make" is run, we see:

      GEN perl/PM.stamp

in the output, even though it is not likely to have changed.
Let's make this recipe completely silent, as we do for other
auto-generated dependency files (e.g., GIT-CFLAGS).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

We force the GIT-BUILD-OPTIONS recipe to run every time
"make" is invoked. We must do this to catch new options
which may have come from the command-line or environment.

However, we actually update the file's timestamp each time
the recipe is run, whether anything changed or not. As a
result, any files which depend on it (for example, all of
the perl scripts, which need to know whether NO_PERL was
set) will be re-built every time.

Let's do our usual trick of writing to a tempfile, then
doing a "cmp || mv" to update the file only when something
changed.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

The rule for "git-instaweb" depends on "gitweb". This makes
no sense, because:

  1. git-instaweb has no build-time dependency on gitweb; it
     is a run-time dependency

  2. gitweb is a directory that we want to recursively make
     in. As a result, its recipe is marked .PHONY, which
     causes "make" to rebuild git-instaweb every time it is
     run.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Teach the index to optionally remember already seen untracked files
to speed up "git status" in a working tree with tons of cruft.

* nd/untracked-cache: (24 commits)
  git-status.txt: advertisement for untracked cache
  untracked cache: guard and disable on system changes
  mingw32: add uname()
  t7063: tests for untracked cache
  update-index: test the system before enabling untracked cache
  update-index: manually enable or disable untracked cache
  status: enable untracked cache
  untracked-cache: temporarily disable with $GIT_DISABLE_UNTRACKED_CACHE
  untracked cache: mark index dirty if untracked cache is updated
  untracked cache: print stats with $GIT_TRACE_UNTRACKED_STATS
  untracked cache: avoid racy timestamps
  read-cache.c: split racy stat test to a separate function
  untracked cache: invalidate at index addition or removal
  untracked cache: load from UNTR index extension
  untracked cache: save to an index extension
  ewah: add convenient wrapper ewah_serialize_strbuf()
  untracked cache: don't open non-existent .gitignore
  untracked cache: mark what dirs should be recursed/saved
  untracked cache: record/validate dir mtime and reuse cached output
  untracked cache: make a wrapper around {open,read,close}dir()
  ...