Add the ability to use keywords in the backout, branch, checkout, cherrypick, and patch commands. Includes some basic regress tests for the new commands, and also some more contrived test cases for 'got log -c[:]keyword[:(+|-)[N]]'. ok stsp

ok jamsek

This begins enabling the use of keywords in got wherever commit ids or references are used, with more work intended to expand support across all such instances (e.g., branch, checkout, etc.), and add more keywords. The keywords ":base" and ":head" can be passed to 'got {diff,log,update} -c' commands as a substitute for the corresponding commit hash id. Keywords and references can also be modified by appending a ':+' or ':-' and an optional integer N to specify by first parent traversal the Nth generation descendant or antecedent, respectively. If N is omitted, a '1' is implicitly appended. tweaks + ok op and stsp

ok jrick

Also document that the commit/import/tag operations are aborted when the user fails to save the log message from the invoked editor. ok jamsek stsp

ok stsp

open_diff_view() has already called view_close() on error. Patch by Josiah Frentsos.

ok stsp

While checking out files, do not attempt to create every directory that exists in the repository tree without paying consideration to their existence on disk. Instead, let functions which add files worry about creating any missing parent directories. This shakes out two other bugs in broken symlink installation code which ended up passing an absolute path to add_dir_on_disk() and forgot to report progress in an unreachable code path that is now getting used. Problem reported by + ok naddy@

backport of b161263abf48e69eb1f05028b633808dfe337357 to cvg

ok stsp@

similar to what we do for loose objects, avoid deleting pack files that were created "too soon" unless -a is given. This prevents races when gotadmin load, got fetch or gotd are installing a new pack file and a concurrent gotadmin clean attempts to remove it. ok stsp