bundle doc: elaborate on rev<->ref restriction

Elaborate on the restriction that you cannot provide a revision that
doesn't resolve to a reference in the "SPECIFYING REFERENCES" section
with examples.

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

Documentation/git-bundle.txt

@@ -144,14 +144,44 @@ unbundle <file>::
 SPECIFYING REFERENCES
 ---------------------
 
-'git bundle' will only package references that are shown by
-'git show-ref': this includes heads, tags, and remote heads.  References
-such as `master~1` cannot be packaged, but are perfectly suitable for
-defining the basis.  More than one reference may be packaged, and more
-than one basis can be specified.  The objects packaged are those not
-contained in the union of the given bases.  Each basis can be
-specified explicitly (e.g. `^master~10`), or implicitly (e.g.
-`master~10..master`, `--since=10.days.ago master`).
+Revisions must accompanied by reference names to be packaged in a
+bundle.
+
+More than one reference may be packaged, and more than one basis can
+be specified.  The objects packaged are those not contained in the
+union of the given bases.
+
+The 'git bundle create' command resolves the reference names for you
+using the same rules as `git rev-parse --abbrev-ref=loose`. Each
+basis can be specified explicitly (e.g. `^master~10`), or implicitly
+(e.g. `master~10..master`, `--since=10.days.ago master`).
+
+All of these simple cases are OK (assuming we have a "master" and
+"next" branch):
+
+----------------
+$ git bundle create master.bundle master
+$ echo master | git bundle create master.bundle --stdin
+$ git bundle create master-and-next.bundle master next
+$ (echo master; echo next) | git bundle create master-and-next.bundle --stdin
+----------------
+
+And so are these (and the same but omitted `--stdin` examples):
+
+----------------
+$ git bundle create recent-master.bundle master~10..master
+$ git bundle create recent-updates.bundle master~10..master next~5..next
+----------------
+
+A revision name or a range whose right-hand-side cannot be resolved to
+a reference is not accepted:
+
+----------------
+$ git bundle create HEAD.bundle $(git rev-parse HEAD)
+fatal: Refusing to create empty bundle.
+$ git bundle create master-yesterday.bundle master~10..master~5
+fatal: Refusing to create empty bundle.
+----------------
 
 OBJECT PREREQUISITES
 --------------------