git-config: do not complain about duplicate entries

git-config: do not complain about duplicate entries

If git-config is asked for a single value, it will complain
and exit with an error if it finds multiple instances of
that value. This is unlike the usual internal config
parsing, however, which will generally overwrite previous
values, leaving only the final one. For example:

  [set a multivar]
  $ git config user.email one@example.com
  $ git config --add user.email two@example.com

  [use the internal parser to fetch it]
  $ git var GIT_AUTHOR_IDENT
  Your Name <two@example.com> ...

  [use git-config to fetch it]
  $ git config user.email
  one@example.com
  error: More than one value for the key user.email: two@example.com

This overwriting behavior is critical for the regular
parser, which starts with the lowest-priority file (e.g.,
/etc/gitconfig) and proceeds to the highest-priority file
($GIT_DIR/config). Overwriting yields the highest priority
value at the end.

Git-config solves this problem by implementing its own
parsing. It goes from highest to lowest priorty, but does
not proceed to the next file if it has seen a value.

So in practice, this distinction never mattered much,
because it only triggered for values in the same file. And
there was not much point in doing that; the real value is in
overwriting values from lower-priority files.

However, this changed with the implementation of config
include files. Now we might see an include overriding a
value from the parent file, which is a sensible thing to do,
but git-config will flag as a duplication.

This patch drops the duplicate detection for git-config and
switches to a pure-overwrite model (for the single case;
--get-all can still be used if callers want to do something
more fancy).

As is shown by the modifications to the test suite, this is
a user-visible change in behavior. An alternative would be
to just change the include case, but this is much cleaner
for a few reasons:

  1. If you change the include case, then to what? If you
     just stop parsing includes after getting a value, then
     you will get a _different_ answer than the regular
     config parser (you'll get the first value instead of
     the last value). So you'd want to implement overwrite
     semantics anyway.

  2. Even though it is a change in behavior for git-config,
     it is bringing us in line with what the internal
     parsers already do.

  3. The file-order reimplementation is the only thing
     keeping us from sharing more code with the internal
     config parser, which will help keep differences to a
     minimum.

Going under the assumption that the primary purpose of
git-config is to behave identically to how git's internal
parsing works, this change can be seen as a bug-fix.

Signed-off-by: Jeff King <peff@peff.net>