git-credential-store: support XDG_CONFIG_HOME

git-credential-store: support XDG_CONFIG_HOME

Add $XDG_CONFIG_HOME/git/credentials to the default credential search
path of git-credential-store. This allows git-credential-store to
support user-specific configuration files in accordance with the XDG
base directory specification[1].

[1] http://standards.freedesktop.org/basedir-spec/basedir-spec-0.7.html

~/.git-credentials has a higher precedence than
$XDG_CONFIG_HOME/git/credentials when looking up credentials.  This
means that if any duplicate matching credentials are found in the xdg
file (due to ~/.git-credentials being updated by old versions of git or
outdated tools), they will not be used at all. This is to give the user
some leeway in switching to old versions of git while keeping the xdg
directory. This is consistent with the behavior of git-config.

However, the higher precedence of ~/.git-credentials means that as long
as ~/.git-credentials exist, all credentials will be written to the
~/.git-credentials file even if the user has an xdg file as having a
~/.git-credentials file indicates that the user wants to preserve
backwards-compatibility. This is also consistent with the behavior of
git-config.

To make this precedence explicit in docs/git-credential-store, add a new
section FILES that lists out the credential file paths in their order of
precedence, and explain how the ordering affects the lookup, storage and
erase operations.

Also, update the documentation for --file to briefly explain the
operations on multiple files if the --file option is not provided.

Since the xdg file will not be used unless it actually exists, to
prevent the situation where some credentials are present in the xdg file
while some are present in the home file, users are recommended to not
create the xdg file if they require compatibility with old versions of
git or outdated tools. Note, though, that "erase" can be used to
explicitly erase matching credentials from all files.

Helped-by: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>
Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Jeff King <peff@peff.net>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Paul Tan <pyokagan@gmail.com>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>