git-am: give better diagnostics when the patch does not apply during --3way

If the user tries to apply a patch that was hand-edited in such
a way that it does not apply to the original file recorded on
its "index" line anymore, we did detect the situation but did
not issue an error message that is specific enough.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-am.sh

@@ -45,6 +45,12 @@ go_next () {
 	this=$next
 }
 
+cannot_fallback () {
+	echo "$1"
+	echo "Cannot fall back to three-way merge."
+	exit 1
+}
+
 fall_back_3way () {
     O_OBJECT=`cd "$GIT_OBJECT_DIRECTORY" && pwd`
 
@@ -52,19 +58,23 @@ fall_back_3way () {
     mkdir "$dotest/patch-merge-tmp-dir"
 
     # First see if the patch records the index info that we can use.
-    if git-apply -z --index-info "$dotest/patch" \
-	>"$dotest/patch-merge-index-info" 2>/dev/null &&
-	GIT_INDEX_FILE="$dotest/patch-merge-tmp-index" \
-	git-update-index -z --index-info <"$dotest/patch-merge-index-info" &&
-	GIT_INDEX_FILE="$dotest/patch-merge-tmp-index" \
-	git-write-tree >"$dotest/patch-merge-base+" &&
-	# index has the base tree now.
-	GIT_INDEX_FILE="$dotest/patch-merge-tmp-index" \
+    git-apply -z --index-info "$dotest/patch" \
+	>"$dotest/patch-merge-index-info" &&
+    GIT_INDEX_FILE="$dotest/patch-merge-tmp-index" \
+    git-update-index -z --index-info <"$dotest/patch-merge-index-info" &&
+    GIT_INDEX_FILE="$dotest/patch-merge-tmp-index" \
+    git-write-tree >"$dotest/patch-merge-base+" ||
+    cannot_fallback "Patch does not record usable index information."
+
+    echo Using index info to reconstruct a base tree...
+    if GIT_INDEX_FILE="$dotest/patch-merge-tmp-index" \
 	git-apply $binary --cached <"$dotest/patch"
     then
-	echo Using index info to reconstruct a base tree...
 	mv "$dotest/patch-merge-base+" "$dotest/patch-merge-base"
 	mv "$dotest/patch-merge-tmp-index" "$dotest/patch-merge-index"
+    else
+        cannot_fallback "Did you hand edit your patch?
+It does not apply to blobs recorded in its index."
     fi
 
     test -f "$dotest/patch-merge-index" &&