convert: drastically speed up git conversions

We would formerly exec git cat-file once for every commit, plus once for
every tree and file we wnated to read.  This switches to using git
cat-file's batch mode, which is much, much, much faster.

Using this new code, converting the git git repo to hg ran in 106
minutes on my machine.  Using the stock mercurial, it required 1239
minutes.  I believe this to be typical of the speedups we will see
form this patch.

hgext/convert/git.py

@@ -46,6 +46,18 b' class convert_git(converter_source):'
                     del os.environ['GIT_DIR']
                 else:
                     os.environ['GIT_DIR'] = prevgitdir
+
+        def gitpipe(self, s):
+            prevgitdir = os.environ.get('GIT_DIR')
+            os.environ['GIT_DIR'] = self.path
+            try:
+                return util.popen3(s)
+            finally:
+                if prevgitdir is None:
+                    del os.environ['GIT_DIR']
+                else:
+                    os.environ['GIT_DIR'] = prevgitdir
+
     else:
         def gitopen(self, s, err=None):
             if err == subprocess.PIPE:
@@ -56,6 +68,9 b' class convert_git(converter_source):'
             else:
                 return util.popen('GIT_DIR=%s %s' % (self.path, s), 'rb')
 
+        def gitpipe(self, s):
+            return util.popen3('GIT_DIR=%s %s' % (self.path, s))
+
     def popen_with_stderr(self, s):
         p = subprocess.Popen(s, shell=True, bufsize=-1,
                              close_fds=util.closefds,
@@ -84,6 +99,12 b' class convert_git(converter_source):'
         self.path = path
         self.submodules = []
 
+        self.catfilepipe = self.gitpipe('git cat-file --batch')
+
+    def after(self):
+        for f in self.catfilepipe:
+            f.close()
+
     def getheads(self):
         if not self.rev:
             heads, ret = self.gitread('git rev-parse --branches --remotes')
@@ -98,9 +119,17 b' class convert_git(converter_source):'
     def catfile(self, rev, type):
         if rev == hex(nullid):
             raise IOError
-        data, ret = self.gitread("git cat-file %s %s" % (type, rev))
-        if ret:
+        self.catfilepipe[0].write(rev+'\n')
+        self.catfilepipe[0].flush()
+        info = self.catfilepipe[1].readline().split()
+        if info[1] != type:
             raise util.Abort(_('cannot read %r object at %s') % (type, rev))
+        size = int(info[2])
+        data = self.catfilepipe[1].read(size)
+        if len(data) < size:
+            raise util.Abort(_('cannot read %r object at %s: %s') % (type, rev))
+        # read the trailing newline
+        self.catfilepipe[1].read(1)
         return data
 
     def getfile(self, name, rev):