git-credential-store: support multiple credential files

git-credential-store: support multiple credential files

Previously, git-credential-store only supported storing credentials in a
single file: ~/.git-credentials. In order to support the XDG base
directory specification[1], git-credential-store needs to be able to
lookup and erase credentials from multiple files, as well as to pick the
appropriate file to write to so that the credentials can be found on
subsequent lookups.

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

Note that some credential storage files may not be owned, readable or
writable by the user, as they may be system-wide files that are meant to
apply to every user.

Instead of a single file path, lookup_credential(), remove_credential()
and store_credential() now take a precedence-ordered string_list of
file paths. lookup_credential() expects both user-specific and
system-wide credential files to be provided to support the use case of
system administrators setting default credentials for users.
remove_credential() and store_credential() expect only the user-specific
credential files to be provided as usually the only config files that
users are allowed to edit are their own user-specific ones.

lookup_credential() will read these (user-specific and system-wide) file
paths in order until it finds the 1st matching credential and print it.
As some files may be private and thus unreadable, any file which cannot
be read will be ignored silently.

remove_credential() will erase credentials from all (user-specific)
files in the list.  This is because if credentials are only erased from
the file with the highest precedence, a matching credential may still be
found in a file further down the list. (Note that due to the lockfile
code, this requires the directory to be writable, which should be so for
user-specific config files)

store_credential() will write the credentials to the first existing
(user-specific) file in the list. If none of the files in the list
exist, store_credential() will write to the filename specified by the
first item of the filename list. For backwards compatibility, this
filename should be "~/.git-credentials".

Helped-by: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>
Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Jeff King <peff@peff.net>
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>