zsh:glog: Fix preview with files that were renamed

Previously when calling `glog -- file` with a file that was renamed
sometime in the history, the preview would just be empty for all commits
before the rename, since it's path didn't exist.

Fix this by checking for empty output and falling back to the full patch
in that case.

This also heavily refactors the code around `$fzf_preview` to make it
more readable.

TODO: It would be nice if git-show would fail in this case instead of
      just printing nothing and returning zero

.config/zsh/autoload/git/glog

@@ -49,39 +49,49 @@ local commit_hash='s/^[^a-f0-9]*([a-f0-9]*).*$/\1/'
 local dateshort='--date=format:%F' # year
 local date="$dateshort %T %z" # year time timezone
 
-local -A fzf_preview
-read -r -d '' <<EOT
+# Use git's pager in the preview window (and with it any special highlighting
+# tool, such as diff-so-fancy)
+local pager
+pager="$(git config --get --default="" core.pager)"
+into_pager="${pager:+|} $pager"
+
+# NOTE: use read for a bit better readability of the code since less needs
+# quoting and formatting is easier
+read -r -d '' get_hash_cmd <<EOT
 	hash="\$(echo -E {} | sed -E "$commit_hash")"; [[ -z "\$hash" ]] ||
 EOT
-fzf_preview[construct]="$REPLY"
-
-read -r -d '' <<EOT
+read -r -d '' show_cmd <<EOT
 	git show "$format" "$date" --color=always --patch-with-stat "\$hash"
 EOT
-fzf_preview[patch]="$fzf_preview[construct] { $REPLY"
-# Get file arguments after (and including) `--` and wrap each in quotes
+show_stat_cmd="${show_cmd/patch-with-}"
+
+local -A fzf_preview
+fzf_preview[patch]="$get_hash_cmd $show_cmd $into_pager"
+fzf_preview[stat]="$get_hash_cmd $show_stat_cmd $into_pager"
+
 # TODO: Display `...` behind patch-stat if the patch touched more files
 # TODO: Use `-O <file>` for 'patch' command with `-- <file>` argument, so that
 #       the passed file is ontop although the full patch is shown
 # TODO: Support -L flag as well (add `-s` for list and `-- <file>` & maybe `-W`
 #       for preview)
-fzf_preview[files_only]="$fzf_preview[patch] ${(@)${@:${@[(ei)--]}}/(#m)*/'$MATCH'}"
-
-# Use git's pager in the preview window (and with it any special highlighting
-# tool, such as diff-so-fancy)
-local pager
-pager="$(git config --get --default="" core.pager)"
-fzf_preview[patch]+="${pager:+ | }$pager; }"
-fzf_preview[files_only]+="${pager:+ | }$pager; }"
-
-read -r -d '' <<EOT
-	git show "$format" "$date" --color=always --stat "\$hash"
+# Get file arguments after (and including) `--` and wrap each in quotes
+read -r -d '' 'fzf_preview[files_only]' <<EOT
+	$get_hash_cmd {
+		{
+			out="\$($show_cmd ${(@)${@:${@[(ei)--]}}/(#m)*/'$MATCH'})"
+			if [[ \$out ]]; then
+				printf '%s' "\$out"
+			else
+				echo "warning: files were renamed. showing full diff"
+				$show_cmd
+			fi
+		} $into_pager
+	}
 EOT
-fzf_preview[stat]="$fzf_preview[construct] { $REPLY; }"
 
 # Put the commit hash into the clipboard
 # (If no known clipboard tool is available, just print it)
-local fzf_copy_command="$fzf_preview[construct] echo -n \"\$hash\""
+local fzf_copy_command="$get_hash_cmd echo -n \"\$hash\""
 if [[ $OSTYPE =~ darwin ]] && (( $+commands[pbcopy] )); then
 	fzf_copy_command+=" | pbcopy"
 elif (( $+commands[xclip] )); then