[PATCH v4] Add code to Mercurial VCS backend to show topic if there is any.

[Manuel Jacob <me@xxxxxxxxxxxxxx>]

"Topics" is an experimental concept in Mercurial that augments the
current branching concept (called "named branches").

For more information, see the not always up-to-date Mercurial Wiki page
https://www.mercurial-scm.org/wiki/TopicPlan.
---
 Doc/Zsh/contrib.yo                               |  3 ++-
 Functions/VCS_Info/Backends/VCS_INFO_get_data_hg | 11 ++++++++++-
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/Doc/Zsh/contrib.yo b/Doc/Zsh/contrib.yo
index 0909cd4f5..b9f48e8af 100644
--- a/Doc/Zsh/contrib.yo
+++ b/Doc/Zsh/contrib.yo
@@ -1328,7 +1328,8 @@ enditem()
 In tt(branchformat) these replacements are done:
 
 startsitem()
-sitem(tt(%b))(The branch name.)
+sitem(tt(%b))(The branch name. For tt(hg), the branch name can include a
+topic name.)
 sitem(tt(%r))(The current revision number or the tt(hgrevformat) style for
 tt(hg).)
 endsitem()
diff --git a/Functions/VCS_Info/Backends/VCS_INFO_get_data_hg b/Functions/VCS_Info/Backends/VCS_INFO_get_data_hg
index cd5ef321d..9f61aeaf4 100644
--- a/Functions/VCS_Info/Backends/VCS_INFO_get_data_hg
+++ b/Functions/VCS_Info/Backends/VCS_INFO_get_data_hg
@@ -5,7 +5,7 @@
 
 setopt localoptions extendedglob NO_shwordsplit
 
-local hgbase bmfile branchfile rebasefile dirstatefile mqseriesfile \
+local hgbase bmfile branchfile topicfile rebasefile dirstatefile mqseriesfile \
     curbmfile curbm \
     mqstatusfile mqguardsfile patchdir mergedir \
     r_csetid r_lrev r_branch i_bmhash i_bmname \
@@ -27,6 +27,7 @@ mergedir="${hgbase}/.hg/merge/"
 bmfile="${hgbase}/.hg/bookmarks"
 curbmfile="${hgbase}/.hg/bookmarks.current"
 branchfile="${hgbase}/.hg/branch"
+topicfile="${hgbase}/.hg/topic"
 rebasefile="${hgbase}/.hg/rebasestate"
 dirstatefile="${hgbase}/.hg/dirstate"
 mqstatusfile="${patchdir}/status" # currently applied patches
@@ -69,6 +70,14 @@ fi
 # If we still don't know the branch it's safe to assume default
 [[ -n ${r_branch} ]] || r_branch="default"
 
+# Show topic if there is any (the UI for this experimental concept is not yet
+# final, but for a long time the convention has been to join the branch name
+# and the topic name by a colon)
+if [[ -f ${topicfile} && -r ${topicfile} && -s ${topicfile} ]] ; then
+    IFS= read -r REPLY < ${topicfile}
+    r_branch=${r_branch}:${REPLY}
+fi
+
 # The working dir has uncommitted-changes if the revision ends with a +
 if [[ $r_lrev[-1] == + ]] ; then
     hgchanges=1
-- 
2.27.0