delta: move some delta chain related computation earlier in deltainfo

They are some more optimization change that will make use of this in the
function. So we retrieve the data earlier.

mercurial/revlogutils/deltas.py

@@ -932,6 +932,21 b' class deltacomputer(object):'
 
     def _builddeltainfo(self, revinfo, base, fh):
         # can we use the cached delta?
+        revlog = self.revlog
+        chainbase = revlog.chainbase(base)
+        if revlog._generaldelta:
+            deltabase = base
+        else:
+            deltabase = chainbase
+        snapshotdepth = None
+        if revlog._sparserevlog and deltabase == nullrev:
+            snapshotdepth = 0
+        elif revlog._sparserevlog and revlog.issnapshot(deltabase):
+            # A delta chain should always be one full snapshot,
+            # zero or more semi-snapshots, and zero or more deltas
+            p1, p2 = revlog.rev(revinfo.p1), revlog.rev(revinfo.p2)
+            if deltabase not in (p1, p2) and revlog.issnapshot(deltabase):
+                snapshotdepth = len(revlog._deltachain(deltabase)[0])
         delta = None
         if revinfo.cachedelta:
             cachebase, cachediff = revinfo.cachedelta
@@ -945,31 +960,14 b' class deltacomputer(object):'
                 delta = revinfo.cachedelta[1]
         if delta is None:
             delta = self._builddeltadiff(base, revinfo, fh)
-        revlog = self.revlog
         header, data = revlog.compress(delta)
         deltalen = len(header) + len(data)
-        chainbase = revlog.chainbase(base)
         offset = revlog.end(len(revlog) - 1)
         dist = deltalen + offset - revlog.start(chainbase)
-        if revlog._generaldelta:
-            deltabase = base
-        else:
-            deltabase = chainbase
         chainlen, compresseddeltalen = revlog._chaininfo(base)
         chainlen += 1
         compresseddeltalen += deltalen
 
-        revlog = self.revlog
-        snapshotdepth = None
-        if deltabase == nullrev:
-            snapshotdepth = 0
-        elif revlog._sparserevlog and revlog.issnapshot(deltabase):
-            # A delta chain should always be one full snapshot,
-            # zero or more semi-snapshots, and zero or more deltas
-            p1, p2 = revlog.rev(revinfo.p1), revlog.rev(revinfo.p2)
-            if deltabase not in (p1, p2) and revlog.issnapshot(deltabase):
-                snapshotdepth = len(revlog._deltachain(deltabase)[0])
-
         return _deltainfo(dist, deltalen, (header, data), deltabase,
                           chainbase, chainlen, compresseddeltalen,
                           snapshotdepth)