export: invoke the file prefetch hook

cmdutil.exportfile() is only called by shelve, mq and patchbomb.  Those are
unlikely to mix with lfs, but it may as well be invoked there for completeness.

mercurial/cmdutil.py

@@ -1624,6 +1624,8 b' def export(repo, revs, basefm, fntemplat'
                             the given template.
         Otherwise: All revs will be written to basefm.
     '''
+    scmutil.prefetchfiles(repo, revs, match)
+
     if not fntemplate:
         _exportfile(repo, revs, basefm, '<unnamed>', switch_parent, opts, match)
     else:
@@ -1632,6 +1634,8 b' def export(repo, revs, basefm, fntemplat'
 
 def exportfile(repo, revs, fp, switch_parent=False, opts=None, match=None):
     """Export changesets to the given file stream"""
+    scmutil.prefetchfiles(repo, revs, match)
+
     dest = getattr(fp, 'name', '<unnamed>')
     with formatter.formatter(repo.ui, fp, 'export', {}) as fm:
         _exportfile(repo, revs, fm, dest, switch_parent, opts, match)

tests/test-lfs-serve.t

@@ -304,6 +304,67 b' Misc: process dies early if a requiremen'
   (see https://mercurial-scm.org/wiki/MissingRequirement for more information)
   [255]
 
+  $ echo 'this is an lfs file' > $TESTTMP/client6_clone/lfspair1.bin
+  $ echo 'this is an lfs file too' > $TESTTMP/client6_clone/lfspair2.bin
+  $ hg -R $TESTTMP/client6_clone ci -Aqm 'add lfs pair'
+  $ hg -R $TESTTMP/client6_clone push -q
+
+  $ hg clone -qU http://localhost:$HGPORT $TESTTMP/bulkfetch
+
+Export will prefetch all needed files across all needed revisions
+
+  $ hg -R $TESTTMP/bulkfetch -v export -r 0:tip -o all.export
+  lfs: assuming remote store: http://localhost:$HGPORT/.git/info/lfs
+  exporting patches:
+  lfs: assuming remote store: http://localhost:$HGPORT/.git/info/lfs
+  lfs: need to transfer 4 objects (92 bytes)
+  lfs: downloading a82f1c5cea0d40e3bb3a849686bb4e6ae47ca27e614de55c1ed0325698ef68de (25 bytes)
+  lfs: processed: a82f1c5cea0d40e3bb3a849686bb4e6ae47ca27e614de55c1ed0325698ef68de
+  lfs: downloading bed80f00180ac404b843628ab56a1c1984d6145c391cd1628a7dd7d2598d71fc (23 bytes)
+  lfs: processed: bed80f00180ac404b843628ab56a1c1984d6145c391cd1628a7dd7d2598d71fc
+  lfs: downloading cf1b2787b74e66547d931b6ebe28ff63303e803cb2baa14a8f57c4383d875782 (20 bytes)
+  lfs: processed: cf1b2787b74e66547d931b6ebe28ff63303e803cb2baa14a8f57c4383d875782
+  lfs: downloading d96eda2c74b56e95cfb5ffb66b6503e198cc6fc4a09dc877de925feebc65786e (24 bytes)
+  lfs: processed: d96eda2c74b56e95cfb5ffb66b6503e198cc6fc4a09dc877de925feebc65786e
+  all.export
+  lfs: found bed80f00180ac404b843628ab56a1c1984d6145c391cd1628a7dd7d2598d71fc in the local lfs store
+  lfs: found a82f1c5cea0d40e3bb3a849686bb4e6ae47ca27e614de55c1ed0325698ef68de in the local lfs store
+  lfs: found cf1b2787b74e66547d931b6ebe28ff63303e803cb2baa14a8f57c4383d875782 in the local lfs store
+  lfs: found d96eda2c74b56e95cfb5ffb66b6503e198cc6fc4a09dc877de925feebc65786e in the local lfs store
+
+Export with selected files is used with `extdiff --patch`
+
+  $ rm -r $TESTTMP/bulkfetch/.hg/store/lfs
+  $ hg --config extensions.extdiff= \
+  >    -R $TESTTMP/bulkfetch -v extdiff -r 2:tip --patch $TESTTMP/bulkfetch/lfs.bin
+  lfs: assuming remote store: http://localhost:$HGPORT/.git/info/lfs
+  lfs: assuming remote store: http://localhost:$HGPORT/.git/info/lfs
+  lfs: downloading bed80f00180ac404b843628ab56a1c1984d6145c391cd1628a7dd7d2598d71fc (23 bytes)
+  lfs: processed: bed80f00180ac404b843628ab56a1c1984d6145c391cd1628a7dd7d2598d71fc
+  */hg-8374dc4052cb.patch (glob)
+  lfs: found bed80f00180ac404b843628ab56a1c1984d6145c391cd1628a7dd7d2598d71fc in the local lfs store
+  */hg-9640b57e77b1.patch (glob)
+  --- */hg-8374dc4052cb.patch	* (glob)
+  +++ */hg-9640b57e77b1.patch	* (glob)
+  @@ -2,12 +2,7 @@
+   # User test
+   # Date 0 0
+   #      Thu Jan 01 00:00:00 1970 +0000
+  -# Node ID 8374dc4052cbd388e79d9dc4ddb29784097aa354
+  -# Parent  1477875038c60152e391238920a16381c627b487
+  -lfs
+  +# Node ID 9640b57e77b14c3a0144fb4478b6cc13e13ea0d1
+  +# Parent  d3b84d50eacbd56638e11abce6b8616aaba54420
+  +add lfs pair
+   
+  -diff -r 1477875038c6 -r 8374dc4052cb lfs.bin
+  ---- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  -+++ b/lfs.bin	Thu Jan 01 00:00:00 1970 +0000
+  -@@ -0,0 +1,1 @@
+  -+this is a big lfs file
+  cleaning up temp directory
+  [1]
+
 #endif
 
   $ $PYTHON $TESTDIR/killdaemons.py $DAEMON_PIDS