Merge branch 'release'

reference loop fixes to be included in 0.26.2

diff --combined NEWS
index 352ce3a73a3ab9deb23736a46c72d144c77fce25,3e431658cbed9aec45a850c72a8cc1afb34dfe5b..736d96023e27f9cdef8ea742ab48618e590d9af7
--- 1/NEWS
--- 2/NEWS
+++ b/NEWS
@@@ -1,3 -1,14 +1,14 @@@
+ Notmuch 0.26.2 (UNRELEASED)
+ ===========================
+ 
+ Library Changes
+ ---------------
+ 
+ Make thread indexing more robust against reference loops
+ 
+   Choose a thread root by date in case of reference loops. Fix a
+   related abort in `notmuch show`.
+ 
  Notmuch 0.26.1 (2018-04-02)
  ===========================
  
@@@ -674,7 -685,7 +685,7 @@@ Fixed constant values so they are not a
    Previously, it was impossible to open writable database handles,
    because `DATABASE_MODE_READ_ONLY` and `DATABASE_MODE_READ_WRITE` were
    both set to zero.
 -  The same issue occured with sort modes.
 +  The same issue occurred with sort modes.
  
  Notmuch 0.22.2 (2016-09-08)
  ===========================
@@@ -1034,7 -1045,7 +1045,7 @@@ Path to gpg is now configurabl
  Emacs
  -----
  
 -Avoid rendering large text attachements.
 +Avoid rendering large text attachments.
  
  Improved rendering of CID references in HTML.
  
@@@ -1075,7 -1086,7 +1086,7 @@@ Documentatio
  Sphinx is now mandatory to build docs
  
    Support for using rst2man in place of sphinx to build the
 -  docmumentation has been removed.
 +  documentation has been removed.
  
  Improved notmuch-search-terms.7
  
@@@ -1454,7 -1465,7 +1465,7 @@@ Command-Line Interfac
  The commands set consistent exit status codes on failures
  
    The cli commands now consistently set exit status of 1 on failures,
 -  except where explicitly otherwise noted. The notable expections are
 +  except where explicitly otherwise noted. The notable exceptions are
    the status codes for format version mismatches for commands that
    support formatted output.
  
@@@ -2664,7 -2675,7 +2675,7 @@@ Turn "id:MSG-ID" links into buttons ass
  
  Add keybinding ('c I') for stashing Message-ID's without an id: prefix
  
 -  Reduces manual labour when stashing them for use outside notmuch.
 +  Reduces manual labor when stashing them for use outside notmuch.
  
  Do not query on `notmuch-search` exit
  
@@@ -2699,7 -2710,7 +2710,7 @@@ mailing list
  nmbug - share tags with a given prefix
  
    nmbug helps maintain a git repo containing all tags with a given
 -  prefix (by default "notmuch::"). Tags can be shared by commiting
 +  prefix (by default "notmuch::"). Tags can be shared by committing
    them to git in one location and restoring in another.
  
  Notmuch 0.9 (2011-10-01)