Propagate --exec-path setting to external commands via GIT_EXEC_PATH

Propagate --exec-path setting to external commands via GIT_EXEC_PATH

Let PATH0=$PATH that was set before the invocation.
Let /foo be a build directory.
Let /pfx be the installation prefix.
Let pfxexecpath=/pfx/libexec/git-core.

The following is going on when 'git --exec-path=/foo gc' is invoked:

1. git sets PATH=/foo:$PATH0 using the path from --exec-path

2. gc execs 'git repack' (note: no dash).

3. Since there is a git in /foo (it's a build directory), /foo/git is
   taken.

4. No explicit exec-path is set this time, hence, this secondary git sets
   PATH=$pfxexecpath:/foo:$PATH

5. Since 'repack' is not a built-in, execv_dashed_external execs
   'git-repack' (note: dash).

6. There is a $pfxexecpath/git-repack, and it is taken.

7. This git-repack runs 'git pack-objects' (note: no dash).

8. There is no git in $pfxexecpath, but there is one in /foo. Hence,
   /foo/git is run.

9. pack-objects is a builtin, hence, in effect /foo/git-pack-objects
   is run.

As you can see, the way in which we previously set the PATH allowed to
mix gits of different vintage.  By setting GIT_EXEC_PATH when --exec-path
was given on the command line, we reduce the confusion.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>