git update-git-for-windows: optionally update from a fork

For the upcoming ARM64 support, as well as for the Microsoft fork of Git
for Windows, it would be helpful if there was a way to update from the
GitHub Releases of the respective fork instead of Git for Windows' own
fork.

To facilitate that, let's introduce support for a
`git-update-git-for-windows.config` file next to the script: if it
exists, treat it as a Git config file and read the `update.fromFork`
value, and then continue with that value inserted into the URL
https://github.com/<fork>/releases.

Example: to ask the updater to look for new versions at
https://github.com/dennisameling/git/releases, run the following command
as administrator:

	git config \
	   -f "$(git --exec-path)/git-update-git-for-windows.config" \
	   update.fromFork dennisameling/git

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

git-extra/git-update-git-for-windows

@@ -174,7 +174,22 @@ update_git_for_windows () {
 		;;
 	esac
 
-	latest_tag_url=https://gitforwindows.org/latest-tag.txt
+	if test -f "$0.config"
+	then
+		fork="$(git config -f "$0.config" update.fromFork)"
+		test -n "$fork" || {
+			echo "Could not find update.fromFork in $0.config" >&2
+			return 1
+		}
+		releases_url=https://api.github.com/repos/$fork/releases
+		latest_tag_url=$releases_url/latest
+		latest_eval='latest=${latest_tag#*\"tag_name\": \"}; latest=${latest%%\"*}'
+	else
+		releases_url=https://api.github.com/repos/git-for-windows/git/releases
+		latest_tag_url=https://gitforwindows.org/latest-tag.txt
+		latest_eval='latest=${latest_tag#v}'
+	fi
+
 	latest_tag=$(http_get $latest_tag_url) ||
 	case $?,"$proxy" in
 	7,)
@@ -190,7 +205,7 @@ update_git_for_windows () {
 		;;
 	esac
 
-	latest=${latest_tag#v}
+	eval "$latest_eval"
 	# Did we ask about this version already?
 	recently_seen="$(get_recently_seen)"
 	test -n "$quiet" && test "x$recently_seen" = "x$latest" && return
@@ -215,7 +230,6 @@ update_git_for_windows () {
 	esac
 
 	echo "Update $latest is available" >&2
-	releases_url=https://api.github.com/repos/git-for-windows/git/releases
 	releases=$(http_get $releases_url/latest) || return
 	download=$(echo "$releases" |
 		grep '"browser_download_url": "' |