Handle git worktrees without relying on --show-superproject-working-tree

Dan James's changes in 8584858 made
git-pre-commit-format work with worktrees, but only if using a recent
enough git which has --show-superproject-working-tree.
Unfortunately, git 2.7.4 doesn't have it and is quite widespread (due to
being in Ubuntu 16.04).

This is based on an initial patch by Charles Baylis (who also helped
with understanding why his original patch was failing some tests).

Author: barisione

git-pre-commit-format

@@ -82,39 +82,35 @@ declare git_test_dir=.
 declare top_dir
 
 while true; do
-    top_dir=$(cd "$git_test_dir" && git rev-parse --show-toplevel) || \
+    top_dir=$(git -C "$git_test_dir" rev-parse --show-toplevel) || \
         error_exit "You need to be in the git repository to run this script."
 
+    # Try to handle git worktree.
+    # The best way to deal both with git submodules and worktrees would be to
+    # use --show-superproject-working-tree, but it's not supported in git 2.7.4
+    # which is shipped in Ubuntu 16.04.
+    declare git_common_dir
+    if git_common_dir=$(git -C "$git_test_dir" rev-parse --git-common-dir 2>/dev/null); then
+        # The common dir could be relative, so we make it absolute.
+        git_common_dir=$(cd "$git_test_dir" && realpath "$git_common_dir")
+        declare maybe_top_dir
+        maybe_top_dir=$(realpath "$git_common_dir/..")
+        if [ -e "$maybe_top_dir/.git" ]; then
+            # We are not in a submodules, otherwise common dir would have been
+            # something like PROJ/.git/modules/SUBMODULE and there would not be
+            # a .git directory in PROJ/.git/modules/.
+            top_dir="$maybe_top_dir"
+        fi
+    fi
+
     [ -e "$top_dir/.git" ] || \
         error_exit "No .git directory in $top_dir."
 
     if [ -d "$top_dir/.git" ]; then
         # We are done! top_dir is the root git directory.
         break
     elif [ -f "$top_dir/.git" ]; then
-        # We are in a submodule or git work-tree if .git is a file!
-        if [ -z "$(cd "$git_test_dir" && git rev-parse --show-superproject-working-tree)" ]; then
-            # The --show-superproject-working-tree option is available and we
-            # are in a work tree.
-            gitdir=$(<"$top_dir/.git")
-            gitdir=${gitdir#gitdir: }
-            topdir_basename=${gitdir##*/}
-            git_test_dir=${gitdir%/worktrees/$topdir_basename}
-            break
-        fi
-        # If show-superproject-working-tree returns non-empty string, either:
-        #
-        #   1) --show-superproject-working-tree is not defined for this version of git
-        #
-        #   2) --show-superproject-working-tree is defined and we are in a submodule
-        #
-        # In the first case we will assume it is not a work tree because people
-        # using that advanced technology will be using a recent version of git.
-        #
-        # In second case, we could use the value returned by
-        # --show-superproject-working-tree directly but we do not here because
-        # that would require extra work.
-        #
+        # We are in a submodule.
         git_test_dir="$git_test_dir/.."
     fi
 done