Check for invalid patch names before acting

[stgit] / t / t1800-import / gnu-diff

diff -Naur old/foo.txt new/foo.txt
--- old/foo.txt 2006-11-11 11:26:18.000000000 +0100
+++ new/foo.txt 2006-11-11 11:25:18.000000000 +0100
@@ -3,6 +3,7 @@
 dobedum
 dobidam
 dobidim
+dabadadash
 dobidum
 dobodam
 dobodim
@@ -20,6 +21,7 @@
 dabedim
 dabedum
 dabidam
+dadadadash
 dabidim
 dabidum
 dabodam