convert: detect removal of ".gitmodules" at git source revisions correctly

Before this patch, all operations applied on ".gitmodules" at git
source revisions are treated as modification, even if they are
actually removal of it.

If removal of ".gitmodules" is treated as modification unexpectedly,
"hg convert" is aborted by the exception raised in
"retrievegitmodules()" for ".gitmodules" at the git source revision
removing it, because that revision doesn't have any information of
".gitmodules".

This patch detects removal of ".gitmodules" at git source revisions
correctly.

If ".gitmodules" is removed at the git source revision, this patch
records "hex(nullid)" as the contents hash value for ".hgsub" and
".hgsubstate" at the destination revision.

This patch makes "getfile()" raise IOError also for ".hgstatus" and
".hgsubstate" if the contents hash value is "hex(nullid)", and this
tells removal of ".hgstatus" and ".hgsubstate" at the destination
revision to "localrepository.commitctx()" correctly.

For files other than ".hgstatus" and ".hgsubstate", checking the
contents hash value in "getfile()" may be redundant, because
"catfile()" for them also does so.

But this patch chooses writing it only once at the beginning of
"getfile()", to avoid writing same code twice both for ".hgsub" and
".hgsubstate" separately.

hgext/convert/git.py

@@ -104,6 +104,8 b' class convert_git(converter_source):'
         return data
 
     def getfile(self, name, rev):
+        if rev == hex(nullid):
+            raise IOError
         if name == '.hgsub':
             data = '\n'.join([m.hgsub() for m in self.submoditer()])
             mode = ''
@@ -155,6 +157,7 b' class convert_git(converter_source):'
         seen = set()
         entry = None
         subexists = False
+        subdeleted = False
         for l in fh.read().split('\x00'):
             if not entry:
                 if not l.startswith(':'):
@@ -171,7 +174,11 b' class convert_git(converter_source):'
 
                 if f == '.gitmodules':
                     subexists = True
-                    changes.append(('.hgsub', ''))
+                    if entry[4] == 'D':
+                        subdeleted = True
+                        changes.append(('.hgsub', hex(nullid)))
+                    else:
+                        changes.append(('.hgsub', ''))
                 elif entry[1] == '160000' or entry[0] == ':160000':
                     subexists = True
                 else:
@@ -182,8 +189,11 b' class convert_git(converter_source):'
             raise util.Abort(_('cannot read changes in %s') % version)
 
         if subexists:
-            self.retrievegitmodules(version)
-            changes.append(('.hgsubstate', ''))
+            if subdeleted:
+                changes.append(('.hgsubstate', hex(nullid)))
+            else:
+                self.retrievegitmodules(version)
+                changes.append(('.hgsubstate', ''))
         return (changes, {})
 
     def getcommit(self, version):

tests/test-convert-git.t

@@ -363,6 +363,23 b' convert sub modules'
 
   $ cd ../..
 
+convert the revision removing '.gitmodules' itself (and related
+submodules)
+
+  $ cd git-repo6
+  $ git rm .gitmodules
+  rm '.gitmodules'
+  $ git rm --cached git-repo5
+  rm 'git-repo5'
+  $ commit -a -m 'remove .gitmodules and submodule git-repo5'
+  $ cd ..
+
+  $ hg convert -q git-repo6 git-repo6-hg
+  $ hg -R git-repo6-hg tip -T "{desc|firstline}\n"
+  remove .gitmodules and submodule git-repo5
+  $ hg -R git-repo6-hg tip -T "{file_dels}\n"
+  .hgsub .hgsubstate
+
 damaged git repository tests:
 In case the hard-coded hashes change, the following commands can be used to
 list the hashes and their corresponding types in the repository: