37150: _git: Autocomplete .. for commit ranges

This patch makes three changes:

- Enable 'git diff o/m<TAB>' to complete to 'origin/master..' with the
  ".." being autoremovable.

  This is implemented by the first hunk.  That hunk is a functional
  subset of Oliver's users/20705, however, the latter hasn't been
  committed.

- Make typing "^" remove the "..", in light of the syntax "foo^..bar".

- Make typing "." _not_ remove the ".." but simply append it, since
  "foo...bar" is a valid and useful syntax.

ChangeLog

@@ -1,5 +1,8 @@
 2015-12-05  Daniel Shahaf  <d.s@daniel.shahaf.name>
 
+	* 37150: Completion/Unix/Command/_git: _git: Autocomplete .. for
+	commit ranges
+
 	* 36957: Completion/Unix/Command/_git: _git: Apply matchspecs to
 	filename completion such as 'git log f/b<TAB>' → 'foo/bar.txt's
 

Completion/Unix/Command/_git

@@ -5623,8 +5623,8 @@ __git_commits () {
 
 (( $+functions[__git_heads] )) ||
 __git_heads () {
-  __git_heads_local
-  __git_heads_remote
+  __git_heads_local "$@"
+  __git_heads_remote "$@"
 }
 
 (( $+functions[__git_heads_local] )) ||
@@ -5815,7 +5815,7 @@ __git_commit_ranges () {
       if [[ ${PREFIX} = (#b)((\\|)\^)* ]]; then
 	compset -p ${#match[1]}
       else
-	suf=( -S .. -r '.@~ ^:\t\n\-' )
+	suf=( -S .. -r '@~ \^:\t\n\-' )
       fi
     fi
     expl=( $* $suf )