feat: AGit push options starting with {base64} are decoded (#8479)

- When specifying push options, tooling like [git-repo-go](https://github.com/alibaba/git-repo-go) encodes pushoption values if they contain newlines or non-ASCII characters as they cannot be easily specified via the CLI otherwise.
- Recognize such base64 encoded values in the pushoptions (those that have the `{base64}` prefix), if the base64 decoding fails we return the original value.
- Resolves forgejo/forgejo#8161

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## Release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Features
  - [PR](https://codeberg.org/forgejo/forgejo/pulls/8479): <!--number 8479 --><!--line 0 --><!--description QUdpdCBwdXNoIG9wdGlvbnMgc3RhcnRpbmcgd2l0aCBge2Jhc2U2NH1gIGFyZSBkZWNvZGVk-->AGit push options starting with `{base64}` are decoded<!--description-->
<!--end release-notes-assistant-->

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8479
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

modules/git/pushoptions/pushoptions.go

@@ -4,6 +4,7 @@
 package pushoptions
 
 import (
+	"encoding/base64"
 	"fmt"
 	"os"
 	"strconv"
@@ -109,5 +110,22 @@ func (o gitPushOptions) GetBool(key Key, def bool) bool {
 
 func (o gitPushOptions) GetString(key Key) (string, bool) {
 	val, ok := o[string(key)]
-	return val, ok
+	if !ok {
+		return "", false
+	}
+
+	// If the value is prefixed with `{base64}` then everything after that is very
+	// likely to be encoded via base64.
+	base64Value, found := strings.CutPrefix(val, "{base64}")
+	if !found {
+		return val, true
+	}
+
+	value, err := base64.StdEncoding.DecodeString(base64Value)
+	if err != nil {
+		// Not valid base64? Return the original value.
+		return val, true
+	}
+
+	return string(value), true
 }