agnoster: Define Powerline chars with escape sequence, and use same character on

all platforms. Add clarifying comments. This makes everything consistent
with the current Powerline-patched fonts definitions.

themes/agnoster.zsh-theme

@@ -7,6 +7,9 @@
 #
 # In order for this theme to render correctly, you will need a
 # [Powerline-patched font](https://github.com/Lokaltog/powerline-fonts).
+# Make sure you have a recent version: the code points that Powerline
+# uses changed in 2012, and older versions will display incorrectly,
+# in confusing ways.
 #
 # In addition, I recommend the
 # [Solarized theme](https://github.com/altercation/solarized/) and, if you're
@@ -27,12 +30,19 @@
 
 CURRENT_BG='NONE'
 
-# Fix odd char on mac
-if [[ `uname` == 'Darwin' ]]; then
-    SEGMENT_SEPARATOR='\ue0b0'
-else
-    SEGMENT_SEPARATOR=''
-fi
+# Special Powerline characters
+
+# NOTE: This segment separator character is correct.  In 2012, Powerline changed
+# the code points they use for their special characters. This is the new code point.
+# If this is not working for you, you probably have an old version of the 
+# Powerline-patched fonts installed. Download and install the new version.
+# Do not submit PRs to change this unless you have reviewed the Powerline code point
+# history and have new information.
+# This is defined using a Unicode escape sequence so it is unambiguously readable, regardless of
+# what font the user is viewing this source code in. Do not replace the
+# escape sequence with a single literal character.
+SEGMENT_SEPARATOR='\ue0b0' # 
+
 
 # Begin a segment
 # Takes two arguments, background and foreground. Both can be omitted,
@@ -73,6 +83,7 @@ prompt_context() {
 
 # Git: branch/detached head, dirty status
 prompt_git() {
+  local PL_BRANCH_CHAR='\ue0a0'         # 
   local ref dirty mode repo_path
   repo_path=$(git rev-parse --git-dir 2>/dev/null)
 
@@ -104,7 +115,7 @@ prompt_git() {
     zstyle ':vcs_info:*' formats ' %u%c'
     zstyle ':vcs_info:*' actionformats ' %u%c'
     vcs_info
-    echo -n "${ref/refs\/heads\// }${vcs_info_msg_0_%% }${mode}"
+    echo -n "${ref/refs\/heads\//$PL_BRANCH_CHAR }${vcs_info_msg_0_%% }${mode}"
   fi
 }