libmakepkg: Use only /etc/makepkg.d/gitconfig

Globally set `GIT_CONFIG_GLOBAL` and `GIT_CONFIG_SYSTEM` so that we're
only loading `/etc/makepkg.d/gitconfig` (if it exists) and no other Git
config files.

Allow injecting another value via `MAKEPKG_GIT_CONFIG`.

Fixes: https://gitlab.archlinux.org/pacman/pacman/-/issues/193

doc/makepkg.8.asciidoc

@@ -247,6 +247,11 @@ Environment Variables
 	Use an alternate config file instead of the +{sysconfdir}/makepkg.conf+
 	default.
 
+**MAKEPKG_GIT_CONFIG=**"/path/to/file"::
+	Use an alternate config file for Git instead of the
+	+{sysconfdir}/makepkg.d/gitconfig+ default. makepkg always prevents Git
+	from loading any other config files.
+
 **PKGDEST=**"/path/to/directory"::
 	Directory where the resulting packages will be stored. Overrides the
 	corresponding value defined in linkman:makepkg.conf[5].

meson.build

@@ -433,6 +433,8 @@ configure_file(
   configuration : substs,
   install_dir : join_paths(SYSCONFDIR, 'makepkg.conf.d/'))
 
+install_emptydir(join_paths(SYSCONFDIR, 'makepkg.d/'))
+
 configure_file(
   input : 'etc/pacman.conf.in',
   output : 'pacman.conf',

scripts/libmakepkg/source/git.sh.in

@@ -28,6 +28,8 @@ source "$MAKEPKG_LIBRARY/util/error.sh"
 source "$MAKEPKG_LIBRARY/util/message.sh"
 source "$MAKEPKG_LIBRARY/util/pkgbuild.sh"
 
+export GIT_CONFIG_GLOBAL=/dev/null
+export GIT_CONFIG_SYSTEM=${MAKEPKG_GIT_CONFIG:-'@sysconfdir@/makepkg.d/gitconfig'}
 
 download_git() {
 	# abort early if parent says not to fetch
@@ -161,7 +163,7 @@ calc_checksum_git() {
 	case ${fragment%%=*} in
 		tag|commit)
 			fragval=${fragment##*=}
-			sum=$(GIT_CONFIG_GLOBAL=/dev/null GIT_CONFIG_SYSTEM=/dev/null git -c core.abbrev=no -C "$dir" archive --format tar "$fragval" | "${integ}sum" 2>&1) || ret=1
+			sum=$(git -c core.abbrev=no -C "$dir" archive --format tar "$fragval" | "${integ}sum" 2>&1) || ret=1
 			sum="${sum%% *}"
 			;;
 		*)