Fix shell globbing bugs

There are several problems leading to the unintentional globbing issue:

Firstly, within `rcup` and `rcdn`, when constructing arguments to pass
to `lsrc`, the _for_ loops over the arguments do not have quoted
variables, leading to globbing.  I have quoted these accordingly.

Secondly, `lsrc` is invoked as follows:

```sh
dests_and_srcs="$(lsrc $LS_ARGS)"
```

When shells use command substitution like this, they go through two stages:

- Word expansion. This is useful because it splits `LS_ARGS` back up
into its constituent strings.
- File name expansion. The side effect of this is to introduce globbing.

You can read more about how this works [here](https://www.tldp.org/LDP/Bash-Beginners-Guide/html/sect_03_04.html#sect_03_04_07).

To fix this, I have passed `lsrc` and its arguments to `eval`.  This
involves quoting the relevant arguments, so:

```sh
for dotfiles_dir in "$DOTFILES_DIRS"; do
  LS_ARGS="$LS_ARGS -d $dotfiles_dir"
done
```

becomes

```sh
for dotfiles_dir in "$DOTFILES_DIRS"; do
  LS_ARGS="$LS_ARGS -d \"$dotfiles_dir\""
done
```

Then `lsrc` is invoked as follows:

```sh
dests_and_srcs="$(eval "lsrc $LS_ARGS")"
```

There is one final non-globbing issue: the parsing of arguments can
introduce extra spaces in the variables, which then trip up the
`dotfiles_dir_excludes` function. For example:

```sh
I) includes="$includes $OPTARG";;
```

introduces a space if `includes` is empty or null.

I have introduced the function `append_variable`, which allows two
variables to be appended without introducing unnecessary whitespace.
Then the additional whitespace is never added in the first place.

Fixes #256.

1 files changed, 9 insertions, 9 deletions

diff --git a/bin/lsrc.in b/bin/lsrc.in
index 6fb6162..eed4c0a 100755
--- a/bin/lsrc.in
+++ b/bin/lsrc.in
@@ -166,7 +166,7 @@ dotfiles_dir_excludes() {
   $DEBUG "dotfiles_dir_excludes $dotfiles_dir"
   $DEBUG "  with excludes: $excludes"
 
-  for exclude in $excludes; do
+  for exclude in "$excludes"; do
     if echo "$exclude" | grep ':' >/dev/null; then
       dotfiles_dir_pat="$(echo "$exclude" | sed 's/:.*//')"
       file_glob="$(echo "$exclude" | sed 's/.*://')"
@@ -259,17 +259,17 @@ handle_command_line() {
     case "$opt" in
       F) show_sigils=1;;
       h) show_help ;;
-      I) includes="$includes $OPTARG";;
-      t) arg_tags="$arg_tags $OPTARG";;
+      I) includes="$(append_variable "$includes" "$OPTARG")" ;;
+      t) arg_tags="$(append_variable "$arg_tags" "$OPTARG")" ;;
       v) verbosity=$(($verbosity + 1));;
       q) verbosity=$(($verbosity - 1));;
-      d) dotfiles_dirs="$dotfiles_dirs $OPTARG";;
+      d) dotfiles_dirs="$(append_variable "$dotfiles_dirs" "$OPTARG")" ;;
       V) version=1;;
-      x) excludes="$excludes $OPTARG";;
-      S) symlink_dirs="$symlink_dirs $OPTARG";;
-      s) never_symlink_dirs="$never_symlink_dirs $OPTARG";;
-      U) undotted="$undotted $OPTARG";;
-      u) never_undotted="$never_undotted $OPTARG";;
+      x) excludes="$(append_variable "$excludes" "$OPTARG")" ;;
+      S) symlink_dirs="$(append_variable "$symlink_dirs" "$OPTARG")" ;;
+      s) never_symlink_dirs="$(append_variable "$never_symlink_dirs" "$OPTARG")" ;;
+      U) undotted="$(append_variable "$undotted" "$OPTARG")" ;;
+      u) never_undotted="$(append_variable "$never_undotted" "$OPTARG")" ;;
       B) hostname="$OPTARG";;
       ?) show_help 64 ;;
     esac