The Creds structure in our "creds" package was first introduced in
commit e8de7a3, where it was part of
a different package in an early pre-alpha version of the Git LFS client.
A single method was defined for the structure in that commit, the
Buffer() method, which formatted lines of key/value pairs to be sent
to the git-credential(1) command.

In commit 3a271b1 of PR #1839 this
method was converted into the current bufferCreds() function, a change
made to avoid exporting the method out of the "lfsapi" package, where
the structure was defined at that time.

We next moved the Creds structure (which was then a simple map of
strings to strings) and its one function into the current "creds"
package in commit c752dcd of PR #3270.
Later, in commits 5d5f90e of PR #5381
and 6783078 of PR #5803 we revised the
Creds structure so its map contains arrays of strings, and added the
IsMultistage() method, so we again have a single method defined for
this structure.

In subsequent commits in this PR we will refactor the bufferCreds()
function to accept and return additional values.  As a first step, in
order to keep our code as consistent as possible, we convert it back into
a method, as it was originally defined, except with the lowercase name of
buffer() so it is not exported outside of the "creds" package.  This
retains the intent of commit 3a271b1
from PR #1839, but uses a naming pattern that is more idiomatic for the
Go language.

As well, we add a test function named TestCredsBufferFormat() to the
accompanying creds/creds_test.go source file.  This test function simply
verifies the behaviour of the former bufferCreds() function that we have
now renamed to the buffer() method.  To permit comparison of the contents
of the method's output buffer with the lines of key/value pairs expected
by the test function, we also add an assertCredsLinesMatch() helper
function which splits and sorts the lines in the buffer and compares
them with the sorted set of expected output lines.