stop putting argv[0] dirname at front of PATH

When the "git" wrapper is invoked, we prepend the baked-in
exec-path to our PATH, so that any sub-processes we exec
will all find the git-foo commands that match the wrapper
version.

If you invoke git with an absolute path, like:

  /usr/bin/git foo

we also prepend "/usr/bin" to the PATH. This was added long
ago by by 231af83 (Teach the "git" command to handle some
commands internally, 2006-02-26), with the intent that
things would just work if you did something like:

  cd /opt
  tar xzf premade-git-package.tar.gz
  alias git=/opt/git/bin/git

as we would then find all of the related external commands
in /opt/git/bin. I.e., it made git runtime-relocatable,
since at the time of 231af83, we installed all of the git
commands into $(bindir). But these days, that is not enough.
Since f28ac70 (Move all dashed-form commands to libexecdir,
2007-11-28), we do not put commands into $(bindir), and you
actually need to convert "/usr/bin" into "/usr/libexec". And
not just for finding binaries; we want to find $(sharedir),
etc, the same way.  The RUNTIME_PREFIX build knob does this
the right way, by assuming a sane hierarchy rooted at
"$prefix" when we run "$prefix/bin/git", and inferring
"$prefix/libexec/git-core", etc.

So this feature (prepending the argv[0] dirname to the PATH)
is broken for providing a runtime prefix, and has been for
many years. Does it do anything for other cases?

For the "git" wrapper itself, as well as any commands
shipped by "git", the answer is no. Those are already in
git's exec-path, which is consulted first. For third-party
commands which you've dropped into the same directory, it
does include them. So if you do

  cd /opt
  tar xzf git-built-specifically-for-opt-git.tar.gz
  cp third-party/git-foo /opt/git/bin/git-foo
  alias git=/opt/git/bin/git

it does mean that we will find the third-party "git-foo",
even if you do not put /opt/git/bin into your $PATH. But
the flipside of this is that we will bump the precedence of
_other_ third-party tools that happen to be in the same
directory as git. For example, consider this setup:

  1. Git is installed by the system in /usr/bin. There are
     other system utilities in /usr/bin. E.g., a system
     "vi".

  2. The user installs tools they prefer in /usr/local/bin.
     E.g., vim with a "vi" symlink. They set their PATH to
     /usr/local/bin:/usr/bin to prefer their custom tools.

  3. Running /usr/bin/git puts "/usr/bin" at the front of
     their PATH. When git invokes the editor on behalf of
     the user, they get the system vi, not their normal vim.

There are other variants of this, including overriding
system ruby and python (which is quite common using tools
like "rvm" and "virtualenv", which use relocatable
hierarchies and $PATH settings to get a consistent
environment).

Given that the main motivation for git placing the argv[0]
dirname into the PATH has been broken for years, that the
remaining cases are obscure and unlikely (and easily fixed
by the user just setting up their $PATH sanely), and that
the behavior is hurting real, reasonably common use cases,
it's not worth continuing to do so.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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