Fix script bug where running from a subdirectory didn't work

This was always intended to work, but didn't because script
subprocesses always used the original command name/path, but this
was after changing directory.

The script already relies on the repository being organized a
particular way, so relying on itself (or another script suitable to
do its work) being at the known location is no worse. This also
makes it unnecessary to guard against the rare {}-in-path case.

This change has the further benefit that if the script is ever
successfully invoked via a Windows-style path with "\" separators
(which can happen on Windows), its subprocesses can still execute.

This also removes an altogether ineffective attempt to support the
rare case of a working tree whole top-level directory name ends in
a newline character, and instead changes the comment to note that
this does work. A few other comments throughout are reworded for
clarity as well.

It may make sense to change the script more deeply than this (even
before replacing part of it with a Rust tool). By sacrificing a
small amount of portability, a loop could be used, as find
recognizes -print0 in GNU/Linux (via GNU findutils), most BSD
systems including macOS, and busybox (thus also Alpine Linux).
Unfortunately a recursive glob, which would be even nicer, is not
supported by versions of bash that ship with macOS.

Author: EliahKagan

etc/copy-packetline.sh

@@ -33,7 +33,8 @@ function indent () {
 function generate_all () {
   local root failures
 
-  root="$(git rev-parse --show-toplevel)/."  # /. in case name ends in newline.
+  # Find the working tree. (NOTE: Wrong if the directory name ends in newline.)
+  root="$(git rev-parse --show-toplevel)"
   cd -- "$root"
 
   if ! test -d gix-packetline/src; then
@@ -53,7 +54,11 @@ function generate_all () {
     fail 'unable to remove target'
   fi
 
-  failures="$(find gix-packetline/src/ -exec "$0" --file {} \; -o -print)"
+  failures="$(
+    find gix-packetline/src/ \
+    -exec etc/copy-packetline.sh --file {} \; \
+    -o -print
+  )"
 
   # If we get here, traversal succeeded, but perhaps some generations failed.
   if test -n "$failures"; then
@@ -74,7 +79,7 @@ function first_line_ends_crlf () {
   # avoid false positives with unexpected characters, or with \r not before \n.
 
   head -n 1 -- "$1" |  # Get the longest prefix with no non-trailing \n byte.
-    od -An -ta |       # Show all bytes symbolically, without addresses.
+    od -An -ta |       # Represent all bytes symbolically, without addresses.
     tr -sd '\n' ' ' |  # Scrunch into one line, so "cr nl" appears as such.
     grep -q 'cr nl$'   # Check if the result signifies a \r\n line ending.
 }
@@ -120,7 +125,7 @@ function generate_one () {
     # Cover this case separately, for more useful error messages.
     fail "source file is symbolic link: $source"
   elif ! test -f "$source"; then
-    # This covers less common kinds of files we can't/shouldn't process.
+    # This covers less common kinds of files we can't or shouldn't process.
     fail "source file neither regular file nor directory: $source"
   elif [[ "$source" =~ \.rs$ ]]; then
     copy_with_header "$source" "$target"
@@ -129,13 +134,6 @@ function generate_one () {
   fi
 }
 
-case "$0" in
-*{}*)
-  # Some "find" implementations expand "{}" even inside a larger argument.
-  fail "can't operate portably with literal {} in command name"
-  ;;
-esac
-
 if { test "$#" -eq 1 && test "$1" = '--all'; } || test "$#" -eq 0; then
   generate_all
 elif test "$#" -eq 2 && test "$1" = '--file'; then