git-prompt: look for user-specific bash completion scripts

The location ~/bash_completion.d/ for such scripts seems to be an
established convention, see e.g.

https://github.com/gradle/gradle-completion#install-manually-1

This closes #247

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

Author: dscho

git-extra/git-prompt.sh

@@ -37,3 +37,14 @@ else
 fi
 
 MSYS2_PS1="$PS1"               # for detection by MSYS2 SDK's bash.basrc
+
+# Evaluate all user-specific Bash completion scripts (if any)
+if test -z "$WINELOADERNOEXEC"
+then
+	for c in "$HOME"/bash_completion.d/*.bash
+	do
+		# Handle absence of any scripts (or the folder) gracefully
+		test ! -f "$c" ||
+		. "$c"
+	done
+fi