tests/test-diff-subdir.t

cache: safer handling of failing seek when writing revision branch cache

If the seek for some reason fails (perhaps because the file is too short to
search to the requested position), make sure we seek to the start and rewrite
everything.

It is unknown if this fixes a real problem that ever happened.

  $ hg init

  $ mkdir alpha
  $ touch alpha/one
  $ mkdir beta
  $ touch beta/two

  $ hg add alpha/one beta/two
  $ hg ci -m "start"

  $ echo 1 > alpha/one
  $ echo 2 > beta/two

everything

  $ hg diff --nodates
  diff -r 7d5ef1aea329 alpha/one
  --- a/alpha/one
  +++ b/alpha/one
  @@ -0,0 +1,1 @@
  +1
  diff -r 7d5ef1aea329 beta/two
  --- a/beta/two
  +++ b/beta/two
  @@ -0,0 +1,1 @@
  +2

beta only

  $ hg diff --nodates beta
  diff -r 7d5ef1aea329 beta/two
  --- a/beta/two
  +++ b/beta/two
  @@ -0,0 +1,1 @@
  +2

inside beta

  $ cd beta
  $ hg diff --nodates .
  diff -r 7d5ef1aea329 beta/two
  --- a/beta/two
  +++ b/beta/two
  @@ -0,0 +1,1 @@
  +2

relative to beta

  $ cd ..
  $ hg diff --nodates --root beta
  diff -r 7d5ef1aea329 two
  --- a/two
  +++ b/two
  @@ -0,0 +1,1 @@
  +2

inside beta

  $ cd beta
  $ hg diff --nodates --root .
  diff -r 7d5ef1aea329 two
  --- a/two
  +++ b/two
  @@ -0,0 +1,1 @@
  +2

  $ cd ..