Repair crash when attempting to export empty patch

When using git apply to generate a diffstat the diff must not be empty
otherwise git exits with a failure that cascades to stg export also
failing.

Also repair importing an empty patch. There was a regression when
splitting the diff from the description which would cause stg import to
fail. The message/diff separator is now removed from the diff string.

Fixes #112

Signed-off-by: Peter Grayson <[email protected]>

Author: jpgrayson

stgit/commands/common.py

@@ -343,7 +343,7 @@ def __split_descr_diff(string):
 
     if m:
         desc = string[: m.start()]
-        diff = string[m.start() :]
+        diff = string.split(m.group(), 1)[1]
     else:
         desc = string
         diff = b''

stgit/lib/git/iw.py

@@ -320,13 +320,16 @@ def apply(self, patch_bytes, quiet, reject=False, strip=None, context_lines=None
             raise MergeException('Patch does not apply cleanly')
 
     def diffstat(self, diff):
-        return (
-            self.run(['git', 'apply', '--stat', '--summary'])
-            .encoding(None)
-            .raw_input(diff)
-            .decoding('utf-8')
-            .raw_output()
-        )
+        if diff:
+            return (
+                self.run(['git', 'apply', '--stat', '--summary'])
+                .encoding(None)
+                .raw_input(diff)
+                .decoding('utf-8')
+                .raw_output()
+            )
+        else:
+            return ''
 
     def changed_files(self, tree, pathlimits=()):
         """Find files changed in worktree with respect to a tree.

t/t4300-export.sh

@@ -66,4 +66,12 @@ test_expect_success 'Export numbered patches with custom extension' '
     grep -e "02-patch-2\.mydiff" export5/series
     '
 
+test_expect_success 'Export series with empty patch' '
+    stg new -m patch-6 &&
+    stg export -d export6 &&
+    test_path_is_file export6/patch-6 &&
+    stg delete $(stg series --noprefix) &&
+    stg import -s export6/series
+    '
+
 test_done