gitk: Add a "Copy commit summary" command
When referring to earlier commits in commit messages or other text, one
of the established formats is
<abbrev-sha> ("<summary>", <author-date>)
Add a "Copy commit summary" command to the context menu that puts this
text for the currently selected commit on the clipboard. This makes it
easy for our users to create well-formatted commit references.
The <abbrev-sha> is produced with the %h format specifier to make it
unique. Its length can be controlled with the gitk preference
"Auto-select SHA1 (length)", or, if this preference is set to its
default value (40), with the Git config setting core.abbrev.
Signed-off-by: Beat Bolli <dev+git@drbeat.li>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>