safe.directory: preliminary clean-up

safe.directory: preliminary clean-up

The paths given in the safe.directory configuration variable are
allowed to contain "~user" (which interpolates to user's home
directory) and "%(prefix)" (which interpolates to the installation
location in RUNTIME_PREFIX-enabled builds, and a call to the
git_config_pathname() function is tasked to obtain a copy of the
path with these constructs interpolated.

The function, when it succeeds, always yields an allocated string in
the location given as the out-parameter; even when there is nothing
to interpolate in the original, a literal copy is made.  The code
path that contains this caller somehow made two contradicting and
incorrect assumptions of the behaviour when there is no need for
interpolation, and was written with extra defensiveness against
two phantom risks that do not exist.

One wrong assumption was that the function might yield NULL when
there is no interpolation.  This led to the use of an extra "check"
variable, conditionally holding either the interpolated or the
original string.  The assumption was with us since 8959555c
(setup_git_directory(): add an owner check for the top-level
directory, 2022-03-02) originally introduced the safe.directory
feature.

Another wrong assumption was that the function might yield the same
pointer as the input when there is no interpolation.  This led to a
conditional free'ing of the interpolated copy, that the conditional
never skipped, as we always received an allocated string.

Simplify the code by removing the extra defensiveness.

Signed-off-by: Junio C Hamano <gitster@pobox.com>