Use << rather than <<- heredoc operator

Because `<<-` only makes a difference when the contents of the here
document are indented with tabs, which was not being done anywhere
that `<<-` was used. See GitoxideLabs#1423 for details.

This also removes some spurious indentation (outside of the here
document) in make_dangling_symlink.sh and changes its blank lines
for readability and to match the style in similar scripts.

gix-dir/tests/fixtures/many.sh

@@ -324,7 +324,7 @@ git init type-mismatch-icase
 git init type-mismatch-icase-clash-dir-is-file
 (cd type-mismatch-icase-clash-dir-is-file
   empty_oid=$(git hash-object -w --stdin </dev/null)
-  git update-index --index-info <<-EOF
+  git update-index --index-info <<EOF
 100644 $empty_oid	D/a
 100644 $empty_oid	d
 EOF

gix-index/tests/fixtures/make_index/v2_icase_name_clashes.sh

@@ -10,7 +10,7 @@ symlink_target=$(echo -n 'X' | git hash-object -w --stdin)
 echo "FILE_? filter=arrow" > .gitattributes
 git add -A
 
-git update-index --index-info <<-EOF
+git update-index --index-info <<EOF
 100644 $content_oid	FILE_X
 100644 $content_oid	FILE_x
 100644 $content_oid	file_X

gix-worktree-state/tests/fixtures/make_dangerous_symlink.sh

@@ -11,7 +11,7 @@ empty_oid=$(git hash-object -w --stdin </dev/null)
 fake_dir_target=$(echo -n 'A-dir' | git hash-object -w --stdin)
 fake_file_target=$(echo -n 'A-file' | git hash-object -w --stdin)
 
-git update-index --index-info <<-EOF
+git update-index --index-info <<EOF
 100644 $empty_oid	A-dir/a
 100644 $empty_oid	A-file
 120000 $fake_dir_target	FAKE-DIR

gix-worktree-state/tests/fixtures/make_dangling_symlink.sh

@@ -4,7 +4,8 @@ set -eu -o pipefail
 git init -q
 
 target_oid=$(echo -n "non-existing-target" | git hash-object -w --stdin)
-  git update-index --index-info <<-EOF
+
+git update-index --index-info <<EOF
 120000 $target_oid	dangling
 EOF
 

gix-worktree-state/tests/fixtures/make_ignorecase_collisions.sh

@@ -10,7 +10,7 @@ symlink_target=$(echo -n 'X' | git hash-object -w --stdin)
 echo "FILE_? filter=arrow" > .gitattributes
 git add -A
 
-git update-index --index-info <<-EOF
+git update-index --index-info <<EOF
 100644 $content_oid	FILE_X
 100644 $content_oid	FILE_x
 100644 $content_oid	file_X

gix/tests/fixtures/make_rev_spec_parse_repos.sh

@@ -101,7 +101,7 @@ git init ambiguous_blob_tree_commit
   # create one tree 0000000000cdc
   git write-tree
 
-  sed -e "s/|$//" >patch <<-EOF
+  sed -e "s/|$//" >patch <<EOF
 diff --git a/frotz b/frotz
 index 000000000..ffffff 100644
 --- a/frotz