31159: git: Pass prefix filter to ls-files even if it matches no files

When a branch or tag name is completed with zsh in a large git repo, the
completion is slow if the given prefix doesn't match a file or directory in
the current working directory. Testing with linux.git, which contains release
tags like v3.9 and a directory virt/:

  git log v<tab>

takes about 0.5 seconds, while

  git log v3<tab>

takes about 25 seconds.

(Timed using zsh 4.3.17, on a fairly slow cpu. zsh from git appears to be
quite a bit faster, but the difference between completing v and v3 is still
large.)

The difference between the two is that v<tab> passes the result of v* to git
ls-files while v3<tab> determines that v3* matches no files, and passes an
empty prefix to git ls-files. So git ls-files lists all files in the repo
and passes that on to _multi_parts.

Making git do the expansion of the * after the prefix lets git ls-files v3*
return an empty list, making _multi_parts job easier.

This does not affect the behavior of git log <tab>, but improves the
performance of partial tag and branch tab-completion in the common case where
file names and tag/branch names don't overlap.

ChangeLog

@@ -1,3 +1,8 @@
+2013-04-20  Torstein Hegge  <hegge@resisty.net>
+
+	* 31159: Completion/Unix/Command/_git: git: Pass prefix filter to
+	ls-files even if it matches no files
+
 2013-04-20  Jun T  <takimoto-j@kba.biglobe.ne.jp>
 
 	* 31281: Completion/Unix/Command/_du: _du: add support for Mac OS

Completion/Unix/Command/_git

@@ -5345,7 +5345,7 @@ __git_files () {
   # TODO: --directory should probably be added to $opts when --others is given.
 
   local pref=$gitcdup$gitprefix$PREFIX
-  files=(${(0)"$(_call_program files git ls-files -z --exclude-standard $opts -- ${pref:+$pref\*} 2>/dev/null)"})
+  files=(${(0)"$(_call_program files git ls-files -z --exclude-standard $opts -- ${pref:+$pref\\\*} 2>/dev/null)"})
   __git_command_successful $pipestatus || return
 
 #  _wanted $tag expl $description _files -g '{'${(j:,:)files}'}' $compadd_opts -