guard against runshell loop

doc/bugs/git-annex:_Argument_list_too_long.mdwn

@@ -33,3 +33,8 @@ Log from the web GUI:
 
 # End of transcript or log.
 """]]
+
+> [[done]]; I have added a guard to runshell to detect when it has
+> started to loop. Although I don't understand how a system could be
+> misconfigured to let that happen, without going far out of your way to 
+> mess it up, it's a failure mode that's worth guarding against. --[[Joey]]

standalone/linux/runshell

@@ -19,6 +19,9 @@ if [ ! -e "$base/bin/git" ]; then
 	echo "** base directory $base does not contain bin/git" >&2
 	exit 1
 fi
+if [ -n "$GIT_ANNEX_STANDLONE_ENV" ]; then
+	echo "** runshell loop detected!"> &2
+fi
 
 # Get absolute path to base, to avoid breakage when things change directories.
 orig="$(pwd)"

standalone/osx/git-annex.app/Contents/MacOS/runshell

@@ -21,6 +21,9 @@ if [ ! -e "$bundle/git" ]; then
 	echo "** bundle directory $bundle does not contain git" >&2
 	exit 1
 fi
+if [ -n "$GIT_ANNEX_STANDLONE_ENV" ]; then
+	echo "** runshell loop detected!"> &2
+fi
 
 # Get absolute path to base, to avoid breakage when things change directories.
 orig="$(pwd)"