[PATCH] am: don't infloop for an empty input file

Today, "git am" surprised me. I mistakenly ran it on an empty file and it went into an infinite loop.

  : > e && git am e

To fix it, I made a failing bourne shell "read" break out of the offending loop. Looking through git-am.sh for other instances, I did find one, but didn't try to address it here.

	    action=again
	    while test "$action" = again
	    do
		gettextln "Commit Body is:"
		echo "--------------------------"
		cat "$dotest/final-commit"
		echo "--------------------------"
		# TRANSLATORS: Make sure to include [y], [n], [e], [v] and [a]
		# in your translation. The program will only accept English
		# input at this point.
		gettext "Apply? [y]es/[n]o/[e]dit/[v]iew patch/[a]ccept all "
may infloop->	read reply
		case "$reply" in

In that case (when someone hits ^D in response to that prompt?), you may want to exit altogether.

In the test addition, I didn't try to handle potentially-inflooping code. In coreutils tests, it's easy (since it includes the timeout program): I would just prefix the command with something like "timeout 10", but the timeout command is not universally available. And besides, the infloop is supposed to be fixed, now.

Here's a patch for the infloop I triggered:
[Noticed with and tested against master v1.7.9.2-262-gba998d3,
 but seems to apply also to maint. ]

-- >8 --

git-am.sh's check_patch_format function would attempt to preview the patch to guess its format, but would go into an infinite loop when the patch file happened to be empty. The solution: exit the loop when "read" fails, not when the line var, "$l1" becomes empty.

Signed-off-by: Jim Meyering <meyering@redhat.com>
---
 git-am.sh     |    2 +-
 t/t4150-am.sh |   10 ++++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)