Git looks for this file in GIT_COMMON_DIR, which is usually the same
as GIT_DIR (.git). However, when searching inside a linked worktree,
.git is usually a file that contains the path of the actual git dir,
which in turn contains a file "commondir" which references the directory
where info/exclude may reside, alongside other configuration shared across
all worktrees. This directory is usually the git dir of the main worktree.
Unlike git this does *not* read environment variables GIT_DIR and
GIT_COMMON_DIR, because it is not clear how to interpret them when
searching multiple repositories.
Fixes#1445, Closes#1446
6f2b79f584