git-gc: do not run git-prune by default.

git-prune is not safe when run uncontrolled in parallel while
other git operations are creating new objects.  To avoid
mistakes, do not run git-prune by default from git-gc.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/git-gc.txt

@@ -8,7 +8,7 @@ git-gc - Cleanup unnecessary files and optimize the local repository
 
 SYNOPSIS
 --------
-'git-gc'
+'git-gc' [--prune]
 
 DESCRIPTION
 -----------
@@ -21,6 +21,21 @@ Users are encouraged to run this task on a regular basis within
 each repository to maintain good disk space utilization and good
 operating performance.
 
+OPTIONS
+-------
+
+--prune::
+	Usually `git-gc` packs refs, expires old reflog entries,
+	packs loose objects,
+	and removes old 'rerere' records.  Removal
+	of unreferenced loose objects is an unsafe operation
+	while other git operations are in progress, so it is not
+	done by default.  Pass this option if you want it, and only
+	when you know nobody else is creating new objects in the
+	repository at the same time (e.g. never use this option
+	in a cron script).
+
+
 Configuration
 -------------
 

git-gc.sh

@@ -4,12 +4,26 @@
 #
 # Cleanup unreachable files and optimize the repository.
 
-USAGE=''
+USAGE='git-gc [--prune]'
 SUBDIRECTORY_OK=Yes
 . git-sh-setup
 
+no_prune=:
+while case $# in 0) break ;; esac
+do
+	case "$1" in
+	--prune)
+		no_prune=
+		;;
+	--)
+		usage
+		;;
+	esac
+	shift
+done
+
 git-pack-refs --prune &&
 git-reflog expire --all &&
 git-repack -a -d -l &&
-git-prune &&
+$no_prune git-prune &&
 git-rerere gc || exit