completion: reduce overhead of clearing cached --options

To get the names of all '$__git_builtin_*' variables caching --options
of builtin commands in order to unset them, 8b0eaa41f2 (completion:
clear cached --options when sourcing the completion script,
2018-03-22) runs a 'set |sed s///' pipeline.  This works both in Bash
and in ZSH, but has a higher than necessary overhead with the extra
processes.

In Bash we can do better: run the 'compgen -v __gitcomp_builtin_'
builtin command, which lists the same variables, but without a
pipeline and 'sed' it can do so with lower overhead.
ZSH will still continue to run that pipeline.

This change also happens to work around an issue in the default Bash
version shipped in macOS (3.2.57), reported by users of the Powerline
shell prompt, which was triggered by the same commit 8b0eaa41f2 as
well.  Powerline uses several Unicode Private Use Area code points to
represent some of its pretty text UI elements (arrows and what not),
and these are stored in the $PS1 variable.  Apparently the 'set'
builtin of said Bash version on macOS has issues with these code
points, and produces garbled output where Powerline's special symbols
should be in the $PS1 variable.  This, in turn, triggers the following
error message in the downstream 'sed' process:

  sed: RE error: illegal byte sequence

Other Bash versions, notably 4.4.19 on macOS via homebrew (i.e. a
newer version on the same platform) and 3.2.25 on CentOS (i.e. a
slightly earlier version, though on a different platform) are not
affected.  ZSH in macOS (the versions shipped by default or installed
via homebrew) or on other platforms isn't affected either.

With this patch neither the 'set' builtin is invoked to print garbage,
nor 'sed' to choke on it.

Issue-on-macOS-reported-by: Stephon Harris <theonestep4@gmail.com>
Issue-on-macOS-explained-by: Matthew Coleman <matt@1eanda.com>
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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