Add Bash and Zsh completion scripts (#22646)

This PR adds contrib scripts for bash and zsh completion.

Simply call:

```bash
source contrib/autocompletion/bash_autocomplete
```

or for Zsh:

```bash
source contrib/autocompletion/zsh_autocomplete
```

Signed-off-by: Andrew Thornton <art27@cantab.net>

---------

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: a1012112796 <1012112796@qq.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

contrib/autocompletion/README

@@ -0,0 +1,17 @@
+Bash and Zsh completion
+=======================
+
+From within the gitea root run:
+
+```bash
+source contrib/autocompletion/bash_autocomplete
+```
+
+or for zsh run:
+
+```bash
+source contrib/autocompletion/zsh_autocomplete
+```
+
+These scripts will check if gitea is on the path and if so add autocompletion for `gitea`. Or if not autocompletion will work for `./gitea`.
+If gitea has been installed as a different program pass in the `PROG` environment variable to set the correct program name.

contrib/autocompletion/bash_autocomplete

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+#! /bin/bash
+# Heavily inspired by https://github.com/urfave/cli
+
+_cli_bash_autocomplete() {
+  if [[ "${COMP_WORDS[0]}" != "source" ]]; then
+    local cur opts base
+    COMPREPLY=()
+    cur="${COMP_WORDS[COMP_CWORD]}"
+    if [[ "$cur" == "-"* ]]; then
+      opts=$( ${COMP_WORDS[@]:0:$COMP_CWORD} ${cur} --generate-bash-completion )
+    else
+      opts=$( ${COMP_WORDS[@]:0:$COMP_CWORD} --generate-bash-completion )
+    fi
+    COMPREPLY=( $(compgen -W "${opts}" -- ${cur}) )
+    return 0
+  fi
+}
+
+if [ -z "$PROG" ] && [ ! "$(command -v gitea &> /dev/null)" ] ; then
+  complete -o bashdefault -o default -o nospace -F _cli_bash_autocomplete gitea
+elif [ -z "$PROG" ]; then
+  complete -o bashdefault -o default -o nospace -F _cli_bash_autocomplete ./gitea
+  complete -o bashdefault -o default -o nospace -F _cli_bash_autocomplete "$PWD/gitea"
+else
+  complete -o bashdefault -o default -o nospace -F _cli_bash_autocomplete "$PROG"
+  unset PROG
+fi
+
+
+

contrib/autocompletion/zsh_autocomplete

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+#compdef ${PROG:=gitea}
+
+
+# Heavily inspired by https://github.com/urfave/cli
+
+_cli_zsh_autocomplete() {
+
+  local -a opts
+  local cur
+  cur=${words[-1]}
+  if [[ "$cur" == "-"* ]]; then
+    opts=("${(@f)$(_CLI_ZSH_AUTOCOMPLETE_HACK=1 ${words[@]:0:#words[@]-1} ${cur} --generate-bash-completion)}")
+  else
+    opts=("${(@f)$(_CLI_ZSH_AUTOCOMPLETE_HACK=1 ${words[@]:0:#words[@]-1} --generate-bash-completion)}")
+  fi
+
+  if [[ "${opts[1]}" != "" ]]; then
+    _describe 'values' opts
+  else
+    _files
+  fi
+
+  return
+}
+
+if [ -z $PROG ] ; then
+  compdef _cli_zsh_autocomplete gitea
+else
+  compdef _cli_zsh_autocomplete $(basename $PROG)
+fi