bisect reset: Allow resetting to any commit, not just a branch

‘git bisect reset’ accepts an optional argument specifying a branch to
check out after cleaning up the bisection state.  This lets you
specify an arbitrary commit.

In particular, this provides a way to clean the bisection state
without moving HEAD: ‘git bisect reset HEAD’.  This may be useful if
you are not interested in the state before you began a bisect,
especially if checking out the old commit would be expensive and
invalidate most of your compiled tree.

Clarify the ‘git bisect reset’ documentation to explain this optional
argument, which was previously mentioned only in the usage message.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-bisect.txt

@@ -20,7 +20,7 @@ on the subcommand:
  git bisect bad [<rev>]
  git bisect good [<rev>...]
  git bisect skip [(<rev>|<range>)...]
- git bisect reset [<branch>]
+ git bisect reset [<commit>]
  git bisect visualize
  git bisect replay <logfile>
  git bisect log
@@ -81,16 +81,27 @@ will have been left with the first bad kernel revision in "refs/bisect/bad".
 Bisect reset
 ~~~~~~~~~~~~
 
-To return to the original head after a bisect session, issue the
-following command:
+After a bisect session, to clean up the bisection state and return to
+the original HEAD, issue the following command:
 
 ------------------------------------------------
 $ git bisect reset
 ------------------------------------------------
 
-This resets the tree to the original branch instead of being on the
-bisection commit ("git bisect start" will also do that, as it resets
-the bisection state).
+By default, this will return your tree to the commit that was checked
+out before `git bisect start`.  (A new `git bisect start` will also do
+that, as it cleans up the old bisection state.)
+
+With an optional argument, you can return to a different commit
+instead:
+
+------------------------------------------------
+$ git bisect reset <commit>
+------------------------------------------------
+
+For example, `git bisect reset HEAD` will leave you on the current
+bisection commit and avoid switching commits at all, while `git bisect
+reset bisect/bad` will check out the first bad revision.
 
 Bisect visualize
 ~~~~~~~~~~~~~~~~

git-bisect.sh

@@ -13,8 +13,8 @@ git bisect skip [(<rev>|<range>)...]
         mark <rev>... untestable revisions.
 git bisect next
         find next bisection to test and check it out.
-git bisect reset [<branch>]
-        finish bisection search and go back to branch.
+git bisect reset [<commit>]
+        finish bisection search and go back to commit.
 git bisect visualize
         show bisect status in gitk.
 git bisect replay <logfile>
@@ -311,8 +311,8 @@ bisect_reset() {
 	}
 	case "$#" in
 	0) branch=$(cat "$GIT_DIR/BISECT_START") ;;
-	1) git show-ref --verify --quiet -- "refs/heads/$1" ||
-	       die "$1 does not seem to be a valid branch"
+	1) git rev-parse --quiet --verify "$1^{commit}" > /dev/null ||
+	       die "'$1' is not a valid commit"
 	   branch="$1" ;;
 	*)
 	    usage ;;