[Tags] Add a tag preview with a custom tagpreview.sh script (#1223)

* Add a tag preview with a custom `tagpreview.sh` script

* [Tags] Respect g:fzf_preview_window

Co-authored-by: Ingo Meyer <IJ_M@gmx.de>
Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

bin/tagpreview.sh

@@ -0,0 +1,60 @@
+#!/usr/bin/env bash
+
+REVERSE="\x1b[7m"
+RESET="\x1b[m"
+
+if [ -z "$1" ]; then
+  echo "usage: $0 FILENAME:TAGFILE:EXCMD"
+  exit 1
+fi
+
+IFS=':' read -r FILE TAGFILE EXCMD <<< "$*"
+
+FILE="$(dirname "${TAGFILE}")/${FILE}"
+
+if [ ! -r "$FILE" ]; then
+  echo "File not found ${FILE}"
+  exit 1
+fi
+
+CENTER="$(vim -i NONE -u NONE -e -m -s "${FILE}" \
+              -c "set nomagic" \
+              -c "${EXCMD}" \
+              -c 'let l=line(".") | new | put =l | print | qa!')" || return
+
+START_LINE="$(( CENTER - FZF_PREVIEW_LINES / 2 ))"
+if (( START_LINE <= 0 )); then
+    START_LINE=1
+fi
+END_LINE="$(( START_LINE + FZF_PREVIEW_LINES - 1 ))"
+
+# Sometimes bat is installed as batcat.
+if command -v batcat > /dev/null; then
+  BATNAME="batcat"
+elif command -v bat > /dev/null; then
+  BATNAME="bat"
+fi
+
+if [ -z "$FZF_PREVIEW_COMMAND" ] && [ "${BATNAME:+x}" ]; then
+  ${BATNAME} --style="${BAT_STYLE:-numbers}" \
+             --color=always \
+             --pager=never \
+             --wrap=never \
+             --terminal-width="${FZF_PREVIEW_COLUMNS}" \
+             --line-range="${START_LINE}:${END_LINE}" \
+             --highlight-line="${CENTER}" \
+             "$FILE"
+  exit $?
+fi
+
+DEFAULT_COMMAND="highlight -O ansi -l {} || coderay {} || rougify {} || cat {}"
+CMD=${FZF_PREVIEW_COMMAND:-$DEFAULT_COMMAND}
+CMD=${CMD//{\}/$(printf %q "$FILE")}
+
+eval "$CMD" 2> /dev/null | awk "{ \
+    if (NR >= $START_LINE && NR <= $END_LINE) { \
+        if (NR == $CENTER) \
+            { gsub(/\x1b[[0-9;]*m/, \"&$REVERSE\"); printf(\"$REVERSE%s\n$RESET\", \$0); } \
+        else printf(\"$RESET%s\n\", \$0); \
+        } \
+    }"