patch: check length of git index header only if integer is specified

Otherwise TypeError would be raised. Follows up d1901c4c8ec0.

mercurial/patch.py

@@ -2184,6 +2184,9 b' def difffeatureopts(ui, opts=None, untru'
                 # the hash config could be an integer (for length of hash) or a
                 # word (e.g. short, full, none)
                 hlen = int(hconf)
+                if hlen < 0 or hlen > 40:
+                    msg = _("invalid length for extendedheader.index: '%d'\n")
+                    ui.warn(msg % hlen)
             except ValueError:
                 # default value
                 if hconf == 'short' or hconf == '':
@@ -2194,9 +2197,6 b' def difffeatureopts(ui, opts=None, untru'
                     msg = _("invalid value for extendedheader.index: '%s'\n")
                     ui.warn(msg % hconf)
             finally:
-                if hlen < 0 or hlen > 40:
-                    msg = _("invalid length for extendedheader.index: '%d'\n")
-                    ui.warn(msg % hlen)
                 buildopts['index'] = hlen
 
     if whitespace:

tests/test-diff-unified.t

@@ -243,6 +243,28 b' Git diff, adding extended headers'
   -a
   +b
 
+  $ hg diff --git --config experimental.extendedheader.index=-1
+  invalid length for extendedheader.index: '-1'
+  diff --git a/f1 b/f 1
+  rename from f1
+  rename to f 1
+  --- a/f1
+  +++ b/f 1	
+  @@ -1,1 +1,1 @@
+  -a
+  +b
+
+  $ hg diff --git --config experimental.extendedheader.index=whatever
+  invalid value for extendedheader.index: 'whatever'
+  diff --git a/f1 b/f 1
+  rename from f1
+  rename to f 1
+  --- a/f1
+  +++ b/f 1	
+  @@ -1,1 +1,1 @@
+  -a
+  +b
+
 Git diff with noprefix
 
   $ hg --config diff.noprefix=True diff --git --nodates