Encrypt to self by default; basic functionality now needs no configuration

Introduces gcrypt.participants "simple" mode which encrypts to self, and
accepts any valid signature by default. No configuration needed for
private repositories.

We also adds  remote.<name>.gcrypt-participants to configure this per
remote.

git-remote-gcrypt

@@ -328,7 +328,8 @@ read_config()
 	local recp_= key_line= cap_= conf_keyring= conf_part=
 	Conf_signkey=$(git config --path user.signingkey || :)
 	conf_keyring=$(git config --path gcrypt.keyring || :)
-	conf_part=$(git config --get gcrypt.participants '.+' || :)
+	conf_part=$(git config --get "remote.$NAME.gcrypt-participants" '.+' ||
+		    git config --get gcrypt.participants '.+' || :)
 
 	# Figure out which keys we should encrypt to or accept signatures from
 	if isnonnull "$conf_keyring" && isnull "$conf_part"
@@ -339,6 +340,14 @@ read_config()
 		--with-colons --fast-list -k | grep ^pub | cut -f 5 -d :)
 	fi
 
+	if isnull "$conf_part" || iseq "$conf_part" simple
+	then
+		Signers="(default keyring)"
+		Recipients="--throw-keyids --default-recipient-self"
+		Goodsig="^\[GNUPG:\] GOODSIG "
+		return 0
+	fi
+
 	for recp_ in $conf_part
 	do
 		key_line=$(gpg --with-colons --fast-list -k "$recp_" | xgrep ^pub)
@@ -662,7 +671,7 @@ EOF
 	fi
 
 	# Generate manifest
-	echo_info "Participants are: $Signers"
+	echo_info "Encrypting to: $Recipients"
 	echo_info "Requesting manifest signature"
 
 	TmpManifest_Enc="$Localdir/tmp_manifest.$$"