completion: cache the path to the repository

After the previous changes in this series there are only a handful of
$(__gitdir) command substitutions left in the completion script, but
there is still a bit of room for improvements:

  1. The command substitution involves the forking of a subshell,
     which has considerable overhead on some platforms.

  2. There are a few cases, where this command substitution is
     executed more than once during a single completion, which means
     multiple subshells and possibly multiple 'git rev-parse'
     executions.  __gitdir() is invoked twice while completing refs
     for e.g. 'git log', 'git rebase', 'gitk', or while completing
     remote refs for 'git fetch' or 'git push'.

Both of these points can be addressed by using the
__git_find_repo_path() helper function introduced in the previous
commit:

  1. __git_find_repo_path() stores the path to the repository in a
     variable instead of printing it, so the command substitution
     around the function can be avoided.  Or rather: the command
     substitution should be avoided to make the new value of the
     variable set inside the function visible to the callers.
     (Yes, there is now a command substitution inside
     __git_find_repo_path() around each 'git rev-parse', but that's
     executed only if necessary, and only once per completion, see
     point 2. below.)

  2. $__git_repo_path, the variable holding the path to the
     repository, is declared local in the toplevel completion
     functions __git_main() and __gitk_main().  Thus, once set, the
     path is visible in all completion functions, including all
     subsequent calls to __git_find_repo_path(), meaning that they
     wouldn't have to re-discover the path to the repository.

So call __git_find_repo_path() and use $__git_repo_path instead of the
$(__gitdir) command substitution to access paths in the .git
directory.  Turn tests checking __gitdir()'s repository discovery into
tests of __git_find_repo_path() such that only the tested function
changes but the expected results don't, ensuring that repo discovery
keeps working as it did before.

As __gitdir() is not used anymore in the completion script, mark it as
deprecated and direct users' attention to __git_find_repo_path() and
$__git_repo_path.  Yet keep four __gitdir() tests to ensure that it
handles success and failure of __git_find_repo_path() and that it
still handles its optional remote argument, because users' custom
completion scriptlets might depend on it.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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