Since 07b2c0eacac (config: learn the "onbranch:" includeIf condition,
2019-06-05), there is a potential catch-22 in the early config path: if
the `include.onbranch:` feature is used, Git assumes that the Git
directory has been initialized already. However, in the early config
code path that is not true.

One way to trigger this is to call the following commands in any
repository:

	git config includeif.onbranch:refs/heads/master.path broken
	git help -a

The symptom triggered by the `git help -a` invocation reads like this:

BUG: refs.c:1851: attempting to get main_ref_store outside of repository

Let's work around this, simply by ignoring the `includeif.onbranch:`
setting when parsing the config when the ref store has not been
initialized (yet).

Technically, there is a way to solve this properly: teach the refs
machinery to initialize the ref_store from a given gitdir/commondir pair
(which we _do_ have in the early config code path), and then use that in
`include_by_branch()`. This, however, is a pretty involved project, and
we're already in the feature freeze for Git v2.23.0.

Note: when calling above-mentioned two commands _outside_ of any Git
worktree (passing the `--global` flag to `git config`, as there is
obviously no repository config available), at the point when
`include_by_branch()` is called, `the_repository` is `NULL`, therefore
we have to be extra careful not to dereference it in that case.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>