extdata: show debug message if external command exits with non-zero status

This isn't fatal because it's quite common for grep to exit with 1. Thanks to
Foozy for spotting this.

mercurial/scmutil.py

@@ -1065,6 +1065,10 b' def extdatasource(repo, source):'
     finally:
         if proc:
             proc.communicate()
+            if proc.returncode != 0:
+                # not an error so 'cmd | grep' can be empty
+                repo.ui.debug("extdata command '%s' %s\n"
+                              % (cmd, util.explainexit(proc.returncode)[0]))
         if src:
             src.close()
 

tests/test-extdata.t

@@ -12,6 +12,7 b' test revset support'
   > filedata = file:extdata.txt
   > notes = notes.txt
   > shelldata = shell:cat extdata.txt | grep 2
+  > emptygrep = shell:cat extdata.txt | grep empty
   > EOF
   $ cat <<'EOF' > extdata.txt
   > 2 another comment on 2
@@ -42,6 +43,12 b' test weight of extdata() revset'
     (symbol '3'))
   3
 
+test non-zero exit of shell command
+
+  $ hg log -qr "extdata(emptygrep)"
+  $ hg log -qr "extdata(emptygrep)" --debug
+  extdata command 'cat extdata.txt | grep empty' exited with status * (glob)
+
 test bad extdata() revset source
 
   $ hg log -qr "extdata()"