Optionally skip linking/copying the built-ins

Optionally skip linking/copying the built-ins

For a long time already, the non-dashed form of the built-ins is the
recommended way to write scripts, i.e. it is better to call `git merge
[...]` than to call `git-merge [...]`.

While Git still supports the dashed form (by hard-linking the `git`
executable to the dashed name in `libexec/git-core/`), in practice, it
is probably almost irrelevant.

However, we *do* care about keeping people's scripts working (even if
they were written before the non-dashed form started to be recommended).

Keeping this backwards-compatibility is not necessarily cheap, though:
even so much as amending the tip commit in a git.git checkout will
require re-linking all of those dashed commands. On this developer's
laptop, this makes a noticeable difference:

	$ touch version.c && time make
	    CC version.o
	    AR libgit.a
	    LINK git-bugreport.exe
	    [... 11 similar lines ...]
	    LN/CP git-remote-https.exe
	    LN/CP git-remote-ftp.exe
	    LN/CP git-remote-ftps.exe
	    LINK git.exe
	    BUILTIN git-add.exe
	    [... 123 similar lines ...]
	    BUILTIN all
	    SUBDIR git-gui
	    SUBDIR gitk-git
	    SUBDIR templates
	    LINK t/helper/test-fake-ssh.exe
	    LINK t/helper/test-line-buffer.exe
	    LINK t/helper/test-svn-fe.exe
	    LINK t/helper/test-tool.exe

	real    0m36.633s
	user    0m3.794s
	sys     0m14.141s

	$ touch version.c && time make SKIP_DASHED_BUILT_INS=1
	    CC version.o
	    AR libgit.a
	    LINK git-bugreport.exe
	    [... 11 similar lines ...]
	    LN/CP git-remote-https.exe
	    LN/CP git-remote-ftp.exe
	    LN/CP git-remote-ftps.exe
	    LINK git.exe
	    BUILTIN git-receive-pack.exe
	    BUILTIN git-upload-archive.exe
	    BUILTIN git-upload-pack.exe
	    BUILTIN all
	    SUBDIR git-gui
	    SUBDIR gitk-git
	    SUBDIR templates
	    LINK t/helper/test-fake-ssh.exe
	    LINK t/helper/test-line-buffer.exe
	    LINK t/helper/test-svn-fe.exe
	    LINK t/helper/test-tool.exe

	real    0m23.717s
	user    0m1.562s
	sys     0m5.210s

Also, `.zip` files do not have any standardized support for hard-links,
therefore "zipping up" the executables will result in inflated disk
usage. (To keep down the size of the "MinGit" variant of Git for
Windows, which is distributed as a `.zip` file, the hard-links are
excluded specifically.)

In addition to that, some programs that are regularly used to assess
disk usage fail to realize that those are hard-links, and heavily
overcount disk usage. Most notably, this was the case with Windows
Explorer up until the last couple of Windows 10 versions. See e.g.
https://github.com/msysgit/msysgit/issues/58.

To save on the time needed to hard-link these dashed commands, with the
plan to eventually stop shipping with those hard-links on Windows, let's
introduce a Makefile knob to skip generating them.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>