Format shell scripts with shfmt.

All changes in this patch were performed automatically with `shfmt` with
configuration flags specified in `.pre-commit-config.yaml`.

In addition to fixing whitespace the roundtrip through shfmt's AST also
transforms command substitutions

    `cmd`
    # becomes
    $(cmd)

and some redirects

    >&2 echo "msg"
    # becomes
    echo >&2 "msg"

testing/external/scripts/find-git-repos

@@ -3,6 +3,6 @@
 # Returns a list of git repositories found in subdirs of the
 # current directory.
 
-for i in `find . -type d`; do
+for i in $(find . -type d); do
     test -e $i/.git && echo $i
 done